Potion stays in effect. As far as the necromancy comment, I guess if you were fighting someone who drank a potion to use against you and you killed them then reanimated them, maybe the potion would still work for the rest of the duration. But I don’t think it would work on a corpse after the fact. Or if you want to really be mean, if a party member took a potion and actually died and you reanimated them instead of raising them I could see that possibly working. Don’t want that potion to go to waste ;) The player, however, might not be too happy.

Picking a pocket in combat definitely at disadvantage and probably high DC. 

doorways are made to be walked through.