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 flavor
So 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.