Hi Poink, welcome to the wonderful, wacky, witless world of being a GM.
I can't comment much on the setting you've chosen, but I will try to address some of your other questions.
I should preface this with the statement that everything I say below is just my own personal style and opinion. It isn't the "right" way to GM and may not be the best way for you. It works for me and has worked for me for decades. But your mileage may vary.
How much you prepare depends entirly on both your skill and your comfort level in "winging it". In general if you are not experienced, preparation is a good thing because you will gain confidence from being able to keep the narrative going. Over time you will probably learn how to keep the game going with less advance preparation and more on the fly decision making.
Having said that, most of the game preparation that is needed is in the area of setting up the setting and the NPCs, which in your case seems like quite a bit of preparation since you are doing a significant conversion. You should have enough maps to keep the party oriented in the world and to provide appropriate tactical battle options. These are two different things and can be two different kinds of maps, but if you plan to use miniatures and battle grids, your tactical battle maps should be detailed enough that you can either place them on the game table, or quickly use them as a reference to draw accurate maps on a dry erase battle mat. Your NPCs and monsters will need to be statted out so that you can quickly manage them in combat, or deal with social interactions such as bluff, diplomacy or sense motive checks.
I have always deliberately avoided having NPCs be part of the party, and I have especially avoided having the NPCs be the most powerful members of the party. The point of the game is to let the players shine, not for them to be backup for a powerful NPC. When I do have NPCs with the party they tend to be there to provide guidance or sometimes to augment healing. They almost never engage in combat and they certainly don't lead the party into combat. Truly influential NPCs who provide your players with motivation, rewards etc should be bit players who only show up when the party needs to know their next steps or pay them a reward. They certainly should never swoop in to save the party except in the direst need because that can make your players simultaneously feel like they are not in control of their destiny, and that they can't be really hurt because they'll be saved by "Gandalf."
Usually I work with my players on their backstory. I provide the broad outline of the world, the politics, the locations of the major cities, countries, etc. but they provide the details like "My character was a poor farmer's son who was left on his own when his father passed away and left him with nothing but an old sword and backpack."
The Pathfinder GM's Guide has excellent guidance for creating both combat and non-combat encounters. I highly recommend you get a copy and read it from cover to cover.
Your idea about arcane/divine rivalry is a fine idea, but the implementatoin of it will be key. Keep in mind that there are some classes that sort of bridge the gap between divine and arcane, and some players might want their characters to multi-class and you don't want to set up situations that make them feel like they can't exercise their own preferences in character creation or advancement.
Good luck, you sort of remind me of me, about 30 years ago... :)
Just a bit of advice on the setting conversion.
Start small. My first campaign was a simple ruined castle on an island in a river just a few miles down the road from a small town. As I was taking the PCs through the ruined castle, between sessions, I was expanding the world chunk by chunk, a mountain range here, a city there, a nation there, etc. By the time they completed the ruined castle (which took them three or four sessions) I had the time to have fleshed out an entire continent with nations, political systems, trade routes, geography, climate, major NPCs, legends and folklore, history, etc.
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Level 11 with double feats.....yikes.
How much do I need to prepare ahead of time? How detailed should it be?
This really depends on what you are trying to accomplish. Is there a structured storyline you are following or are you going to provide a sandbox? In sandbox as GM you personally need to know your world. Be ready to answer PC questions. Helps to work on improv for SB gaming. With a storyline it helps to feed as much info as possible to your players. This will help them understand their role.
For homebrewing I tend to find sand boxing easier because of my break down above. You can always fly by the seat of your pants on details which opens up time for you to work on encounters. I'd expect to spend double the time on a storyline as I do a sandbox. Take this with a grain of salt somebody will come along and say the exact opposite. It comes down to experience so you really wont know until you dive in.
How powerful/influential should NPCs be that are with the party for significant periods of time?
For me anytime an NPC is with the party for a significant time is probably too long. Another personal opionion here but DMPC is a huge no-no in my circle. I dont mind powerful and influential NPCs as long
as they dont steal the spotlight or railroad the PCs. This is my two coppers on both sides of the screen.
Should I create the backstory for my player's characters, or should they? I have created a world where not everything is the same as outlined in the books, so it would be difficult for them to explain why they are in a part of the world far away from where their race resides. See spoiler above for details.
Why not do both? Mock up some back-stories for characters and give the players options to either run with one of your ideas or to make up their own. Your mock ups can serve as a guide for what to expect from the game.
How do I create a balanced skill-check and combat-encounter campaign? Our previous campaigns never really used any skills beyond the combat-oriented ones, so I really want to use them here
Read the skill descriptions closely. Think of ways to incorporate them into the game. Tracking an enemy you are looking for; use survival. Decipher a strange language; use linguistics. Recall some piece of trivial knowledge about about one of the races that gives you an edge in dealing with them; use knowledge (insert here). Find a way to break in or out of some lace you are not supoposed to be; use Disable device.
I want to have one society where there is only divine magic and arcane is scorned, and the reverse for the other. I also want to create enemies that are susceptible to arcane, but not divine, magic. Is this feasible/a good idea?
Sounds neat but it also sounds like a lot of work. One easy fix might be to give spell resistance against spell type so they are not completely immune to divine/arcane but have a good chance at avoiding the effects.
One last piece of advice for you, be sure to keep an eye on your players. Sometimes a GM can really get carried away with their homebrew idea but the players not so much. You dont want to put in a ton of effort if the players are just meh about your game. Also, be sure to give them some things they really desire to help make it a group activity. Dont take it personal if somebody loses interest in your idea.
I'm not very familiar with Starcraft, but I think it has like space ships and lasers a such (right?). You may have to think this through a lot, as it will change the types of treasure, encounters, and enemies significantly. I'm not trying to discourage you, and I think it's a great idea, but make sure you think through how and if the magic and technology interact at all. I also recommend u look at the Numeria entry in the inner sea guide, as well as the advanced technology section.
I second everything that AD said as well
Edited: I meant to say Numeria
•How much do I need to prepare ahead of time? How detailed should it be?
This is always a sliding scale according to style and preferences. When I run my own material I prep alot less then when I use published material for instance. A friend of mine when he runs games preps ALOT more then I do.
The biggest element of this is going to be your setting. It will take a fair amount of time to incorporate all the starcraft elements you want into pathfinder, regardless of hte level of conversion. After that it will depend.
Some people run things off the cuff and just use things like the bestiary, npc codex and other monster books to quickly pull together encounters on the fly as the players get into various situations. Other people plan detailed encounters, spending hours pairing monster/npc abilities with terrain, and situations. In time you'll figure out where you fit on that scale. But it will take practice. I would say that more is usually better then less, I've never had a game go badly because I prepared too much.
•How powerful/influential should NPCs be that are with the party for significant periods of time?
Not very, you dont want the players to take a back seat to the npcs. Even if you really want someone to be important, let it be because they are important to the story, not because they are enacting the story. For instance, a prince has been dethroned and the players are helping him get back on track. He might give the party inside information, or be the inspiration for the uprising they will start among the populace, but he SHOULDNT be the one to take down his backstabing brother, or lead the people in the revolt. The party should be on center stage as much as possible.
•Should I create the backstory for my player's characters, or should they? I have created a world where not everything is the same as outlined in the books, so it would be difficult for them to explain why they are in a part of the world far away from where their race resides. See spoiler above for details.
Always let them create the backstory. If they dont have enough information to create backstories, you need to find a way to give it to them. The backstory wont mean anything if they dont have the knowledge to act in your world. Either set some time aside to work with them on their backstories, or create some handouts on the world. Traits are a great starting point also to add information and give players something to work with. Creating a bunch of campaign traits (see the players guides for the various adventure paths by paizo for ideas there).
•How do I create a balanced skill-check and combat-encounter campaign? Our previous campaigns never really used any skills beyond the combat-oriented ones, so I really want to use them here
Include situations that are not resolved by killing people, or at least better resolved by not killing people. For instance maybe the party in the aformentioned restore the prince scenario needs to convince a few noble families to join their side of the fight. Then encourage them to seek out information (knowledge nad gather information[diplomacy] checks) and then use it try to convince the noble houses to help (diplomacy, bluff, sense motive).
Investigation plots can also help. Figuring out who killed the young merchant isnt solved by fighting someone. Perception checks, knowedge/diplomacy checks, and survival (tracking) checks do that.
Also make sure you give out rewards for such non-combat encounters the same as you would combat encounters.
•I want to have one society where there is only divine magic and arcane is scorned, and the reverse for the other. I also want to create enemies that are susceptible to arcane, but not divine, magic. Is this feasible/a good idea?
First part, sure, second part, probably not. The unified nature of magic is there not for setting reasons, but for balance reasons. It would be like making a creature immune to swords but not axes. Stuff like spell resistance exists sure, but it shouldnt only apply to certain magic users. That leaves people out of the equation in an encounter, which is usually a bad idea.
and I agree with most of the other things you said. Concerning the vulnerability to different types of magic, though, I would offer the following question: how is it any different than being susceptible to bludgeoning versus slashing damage, et cetera?
2 things make it different. First off, nothing is invulnerable to slashing or bludgeoning. There is damage reduction, making other kinds less effective, but that is not the same as susceptable vs not susceptable.
Second, if something has dr slashing the fighter can switch weapons. A well prepared fighter has a weapon of each type on hand for such occassions. A wizard cannot have divine magic on hand for those monsters that are not vulnerable to arcane spells.
Even though Pathfinder does not, by default, make this distinction for the purposes of balance, it makes sense to believe that divine magic would be at least somewhat different than arcane, or else you would be implying that secular magic users would have access to the same power that the gods/their followers do, albeit on a much smaller scale.
They do, you are not required to follow a God to be a cleric you can be a cleric of an ideal or an oracle, and there are wizards of just about every faith. Regardless of faith you have access to the same power. The power's source is different, but the way that power acts is the same, other then the spells being different.
I am very aware of the balance issues presented by such a change, but it serves as both a unique and challenging obstacle to this party in my opinion. Currently there are no divine spellcasters, only an arcane one, and I plan on making some bosses a long ways down the storyline invulnerable to divine magic for story purposes, but very susceptible to arcane, so it looks like it will work out in that player's favor. Still, I very much appreciate your concern, and it may very well turn out to be a bad idea on my part to do this.
It doesnt present challenges, it removes characters from play. The fact that it might come up due to party composition (though you might want to address the fact that the party has no divine caster, that can be a real issue), it still means if some guy is invulnerable to x kind of magic, x kind of magic user sits out the encounter. Any time a player is completely removed from the encounter its a bad result.
An analogy would be lets say we are playing basketball. I want to challenge my friend. We make this rule he can only shoot from outside the key (no layups), he is now further challenged. What you are suggesting is he isnt allowed to shoot. Thats not a challenge, thats virtual non-participation.
Another question that I seek advice on: If I have planned certain significant events in this universe (significant being nation-wide, possibly turn-of-the-tide kind of events), should I always have the PCs be the driving force behind those events? The way I have it thought out now that would be very difficult without major changes to the story, and I have those events happening due to NPCs that may be outside the influence of the PCs. This, to me, is the more realistic approach, being as that PCs can't be everywhere at once, and they are not the only players in town, so to speak. In my mind, if I can create...
Things can happen without the players. The only thing you should keep a handle on is the fact that the players should feel like they can have a hand in those events if they do make the effort and are present for the events. Basically, players should feel like they have agency (some influence over the events that surround them).
I always recommend Gamemastering by Brian Jamson. I don't agree with everything he says (he doesn't like dungeoncrawls), but Jamson's guide the best Gamemastering guide I've ever seen. He stresses the importance of involving the player characters in the story, creating a cohesive group of players, and GMing with flexibility and as little preparation as possible.
How much do I need to prepare ahead of time? How detailed should it be?
Short Answer: Bare minimum preparation.
Long Answer: It may depend on game structure (linear story? hexcrawl? dungeoncrawl?), but generally you should spend as little time as possible preparing because players have the power to change the direction of the campaign. Preparing games efficiently is a skill that takes time to develop. I began spending over 6 hours preparing (not a good thing), and now I can prepare for a session in only maybe one or two hours.
How powerful/influential should NPCs be that are with the party for significant periods of time?
Generally, you should never force the party to have a powerful NPC tag along. The PCs should be the star, not your NPC. Even if the NPC doesn't do anything, the players may get resentful.
This isn't to say that it can't be done. My campaign has an NPC serving as a permanent party member at the request of the players. However, this is a very unusual circumstance and she's much weaker than the heroes.
If a powerful NPC tags along, it should be at the request of the players or be brief and lead to a good pay off (like an NPC leading the party on a mission only to get killed in action and leave the party stranded).
Should I create the backstory for my player's characters, or should they?
They should do it. Encourage your players to develop compelling characters with good backstories. You want them to have backstories you can work with and involve the PCs into the campaign story. As a GM, it becomes difficult to motivate PCs to do anything if they're bland and generic. If they aren't sure, then work closely with them to develop one they like.
How do I create a balanced skill-check and combat-encounter campaign?
Most skill encounters involve overcoming obstacles, disabling traps, talking to NPCs, or solving puzzles. With exception to traps and obstacles that have a set CR, you usually consider them an encounter CR = APL.
For combat, follow the Gamemastering section in the CRC. Decide your CR, find the XP for that CR, and divide that XP among enemies. For standard fights, I use CR = APL - 2. For challenging fights, I use CR = APL. For campaign bosses, I use CR = APL + 2.
I want to have one society where there is only divine magic and arcane is scorned, and the reverse for the other. I also want to create enemies that are susceptible to arcane, but not divine, magic. Is this feasible/a good idea?
This could work. You could give them a bonus/penalty on their saves against such magic.
I must warn you. You're in for a tough campaign. Homebrew campaign setting with 11th level characters is a tough start for a new GM.