1. Do named NPC's respawn if I kill them? If so, how long does it take for them to respawn?
I dont think so no. some NPC's that are key characters & attached to an important quest line cannot. But others can, but they dont respawan, dead is dead.
2. When I sneak, does my follower sneak too and have my same skill level at it?
They sneak too yes, but no they dont have the same level of sneak & when the enemy is close they normally attack them right away. From what i played.
3. In Oblivion you could only sell stolen items to certain people, is it the same here?
Yes, you can only sell stolen items to fences. There are perks in Speechcraft though, that allows you to sell stolen goods to anyone. Investor + Fence.
4. Should I dual weild or just use a dagger and shield?
It's upto you, whichever you prefer.