What sort of characters are in the adventure, and what would get them interested in this jewel? Would seeing a comet or experiencing weird weather be enough to get them to go find this thing? Perhaps one of the spellcasters is researching these strange corrupted beings and is trying to find the source, or the group could be off doing some completely separate task when all of a sudden a wild man runs out of the forest and attacks them, and they find this gem on his body.
Also, once this gem is introduced, how is the story going to wrap around it? I need more information if I'm to help you out with this.
Perhaps a benevolent but powerful npc wizard or cleric has somehow gained possession of the jewel, and enlists the adventurers to research a way to destroy or contain it. Likely at some stage they end up fighting their insane former employer unless they can find a way to save him.
Corruption and taint seem more loke the work of Evik outsiders to me. Proteans would be focused on making things ubruly and destructive, like wild thunderstorms, sextions of land floating into the sky, and creatures mutating randonly (equally likely to be ebenficial and hurtful). A Silent Hill sort of corruption would be most lije the work of Daemons, who want to destroy life out of a contempt for it, while Proteans sinply want to return the multiverse to its original, chaotic state.
Also, be wary of calling any part of your story "trivial". If you create quests with that thought, your players will find them trivial as well, and that is bad. Not everything they do will be as dangerous or epic, but it should all feel important.
Think about the adventure as a three act structure, and dont worry too much about planning details until you get close to them. An exanple:
Prologue: Players are called on by a summoner friend they all have in common who thinks he found something important. When they get there, he's been murdered and the party has to put down his Unfettered Eidolon (a creature you can make as powerful or weak as needed). The object he spoke of is jowhere to be found.
Act One: What happened to him, and where is the gem? Players gather clues that lead to rumors of a secret cult of chaos that works in the shadows beneath the city, who may have been enemies of the summoner. Players journey down into the undercity (encountering some an ooze and mayve a warren of ratfolk along the way) and eventuakly encounter the cultists, defeat their leader and steal the gem, but not before the leader tells them its too late.
Act Two: The players try to figure out what this object is. They go on a brief journey to find the summoner's old apprentice, who tells them it is tied to the proteans. Ehile on this journey, the notice the weather and landscaoe beginning to change and become more wild. The only way to destroy it is to find an Inevitable (LN outsider) to destroy it. They are presented with several options on how to go about it.
Act Three: The adventure becomes a race against time as the world becomea more and more chaotic. Voidworms start appearing frequently, and once on their journey thy encounter a stronger protean. By the time they find the Inevitable, the world is a mess. They enliat his help, but not before a powerdul protean is unleashed on the material plane. With the Inevitabke's help, the heroes must defeat the powerful outaider.
Obky 2 pcs? You'll need to make an NPC or to to accompany them temprarily. A devoted caster and melee isnt well rounded enough to take on any real threats. Throw in a wilderness guide or friend of one of the other NPCs to fill things out. Yoy'll need sone who can heal a little at least. A monk can deal decent damage and take hits okay, but without a healer they're both screwed.
You could even introduce some non-humanoid conpanions, low CR outsiders that want to foil the protean's plans.
Gestalt rules for 2 players - even if they have an eidolon / animal companion that will barely bring the action economy into line for the norm.
Oh, as far as pacing - do you want an 'action movie' vibe or a slow build into horror and madness? The former, just set the scene and have at it!
They travel with the oracle and her 'posse'. The oracle, an archeologist bard, a brute of a thief, a paladin, a barbarian, and probably a summoner. (for the idea above).
A lot of the 'quests' by the oracle will revolve around an important decision. Which npc to bring?
They all have plot-related info as well, that they'll only discuss when they're alone with the pcs (and asked).
One is a spy for a lawful neutral sect, one is chaotic neutral and wants the gem for himself, one is a former enemy of the oracle that was permenantly charmed/dominated, etc (hello, antimagic field/dispel magic)
The pcs will level very fast up to level 6-8 or so, and then they have a shot at finding out the oracle's intentions.
There will also be times where the group as a whole is attacked,and the party seperated.
That is a lot of NPCs to be following around the party. Be careful of that, having so many individual characters in a combat situation can really slow things down, and end up with your players waiting for 5 minutes to take their 30 second turn while the GM controls a host of different characters/creatures and essentially plays with himself. Even with a very experience GM, it can bog down the flow of turns a lot.
If I were you I would restrict it to 1-2 character that participate in combat and maybe a couple non-combat NPCs if you really want a bigger group (an ordinary blacksmith or archaeologist with no PC class levels).
I'm glad to hear that. My one concern is making sure you players don't start to feel resentment toward these folks if they seem perfectly capable of defending the group but keep running away, or wondering why they travel with the party but are conveniently absent during battles. You can avoid this if you frame everything right, but it's something to keep in mind. Also, if one of your players is playing a diplomatic character, be aware he might try to convince the posse members to fight him him.
If he happens to roll crazy high on a Diplomacy check to get one of them to accompany the group and the character still doesn't agree, the player might feel like you're blocking him and wonder why the posse is there in the first place.
Make sure you aren't trying to force your own characters into a story that should be for and about the PCs.
Hold on, why would they leave when they noticed the oracle's alignment? Is she evil? If she's evil, why is she being accompanied by a paladin?
I'm a little confused, I think you left some important details out. The paladin is LG, but he's attacking the players and using shadow conjuration (which a paladin does not have access to)?
Why is the oracle just killing this person outright? Are all of the players okay with this, especially since his alignment detected as good and he was clearly udner soem sort of compulsion? Don't put the players in a position where their characters have to do something that goes against their alignment. It will either hurt roleplaying (by forcing them to act a certain way) or or hurt the adventure (by making a player go against the group).
Where are the other "posse" members during this scene? Did they simply vanish for the duration of the fight? They're all player classes that can handle themselves in a fight.
Videogames can be a source of inspiration, but be wary of approaching Pathfinder with a videogame mentality. You can't have npcs and characters pop in and out for "cinematics". If a player asks "Where's the barbarian?" and you have to say "He's disappeared for the fight", you are going to run into problems.
I think you might be trying to put too many plot elements into one adventure.
The gem alone was a great adventure seed you could use to make a concise, compelling adventure. The evil-shifting Oracle that convinces the party that the good guys are bad is another. The shadow paladin is kind of odd... as a paladin he would be opposed to using powers that come from a chaos gem.
Since you're new at this, you should really stick with something simple. Do simple adventures with relatively little planning, giving you room to practice improvising and making an adventure that fits your players more. You seem like you're trying to plan everything out meticulously, which is a bad idea. The more you plan, the more disappointed you'll be when your players decide to go off the rails (which WILL HAPPEN if you make rails) and the less prepared you will be to improvise. It's also very difficult to keep track of all these characters and details when you're new.