Have you looked at wayang as a race option? They can steal fun spells as their favored class bonus and being small is an advantage for a full caster. They also have the +1 DC race option for shadow spells. The stat bonuses are perfect as well
Crafting can wait until you get a grasp on whether the campaign is slow-paced enough for it to be practical. Improved familiar can wait until 7th level probably. Shadow grasp has 2 prereqs so you could reasonably get those at levels 1 & 3 then get shadow grasp at 5, decide whether to get improved familiar at 7, then go into crafting or other things at 7 or afterwards.
There's no witch archetype which is particularly about shadow or darkness oddly enough. You could take the base witch or go into invoker perhaps.
Best (improved) familiar for what? I mean, various familiars have different advantages, but without some idea what you want to use the familiar for that's an impossible question to answer.
So I am building a shadow witch for a reign of winter campaign and I am having trouble deciding some of the aspects of it.
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1. Race: I have limited it down to either drow or fetchling. Drow has the cool shadow caster and nobility feats while fetchling has the +1 DC bonus and shadowy flavorSo I would absolutely recommend against both of those above classes, as neither as an int bonus and those are all important as a witch. However, it is not a total loss, as coincidentally half elves happen to be wonderful for what you are going for, and thematically a half elf drow is pretty close to a drow, but also miles better mechanically. Half elf drow can also pick up an alternate racial trait to get dark vision, which is highly recommended having for the shadow grasp tree. the also get skill focus for free which is amazing for a reason I'll touch on below, and they also get paragon surge if your DM allows that which you can use to pick the extra hex feat for a ton of versatility. If you absolutely must take fetching or drow, I think they are roughly equal. Wayang would be another amazing race with tons of shadow flavor for you if you haven't looked into them.
2. Archetype: I do not know what witch archetype would be good for a shadow witch
Anything decent will do good, though I think the base witch is good enough here, though admittedly I have not looked into witch archetypes that much.
3. Feats: I know I eventually want to get the shadow grasp feat tree and want to go into crafting, and maybe get an improved familiar. But I don't know what order to get these feats in
I would turbo the shadow grasp tree as hard as you can. by the time you get second level spells you can cast umbral arcane mark for permanent darkness and by the time you hit 5 you can get shadow grasp for permanent aura of shadowy entanglement around you. Just make sure you have darkvision and this is amazing. You'll have perma concealment against anything without darkvision and even against darkvision you get entanglement aura. Additionally, since you are taking so many metamagic feats early this makes you the perfect candidate for a one level dip in loremaster, which allows you to take the new super strong feat secret of magical discipline, which you can use to spontaneously cast literally any spell in the game you have slots for. And you can get this as early as level 7! If you go with half elf this further satisfies the skill focus requirement of loremaster, so you don't even have to take that. https://www.d20pfsrd.com/feats/general-feats/secret-of-magical-discipline/
4. Familiar: What familiar is best and what should the improved one be. Also, should I use an archetype.
for normal familiar I recommend a greensting scorpion with the protection archetype. this will give you a huge bonus to initiative, extra AC vs attacks, and at lvl 5 an extra health pool equal to half your current HP. it also has a lot of AC, good stealth, and enough dex to make bodyguard work amazing. For an improved familiar, I recommend a fairy dragon or imp with a huge bonus to use magic device who flies around using wands, throwing tanglefoot bags, and the like.
You want to have lots of shadows around? Does your party work well with this? Could be very important.
Both the base witch and Winter Witch archetype are good. Other archetypes need a lot of TLC to work.
For crafting, you can have your Valet familiar aid you do it in half the time. But few archetype work with Improved familiars.
If you go with an Imp, look at the Imp Consular. You need to be CL8, but it gives your imp 50' telepathy and the ability to appear as a Small or Tiny animal. But you are required to be LE.
Other Improved Familiars that might do you good: Homunculus, Nycar, Voidworm, and Wysp.
/cevah