Will hostile NPCs remain hostile forever (after theft)

It seems to me that once you pick up an item it becomes yours. The only issue is if someone spots you picking up an owned item; that's when the trouble starts.

So yes, you can steal the potion and use it for the quest.

You can kill almost anything except main quest givers. Side quest givers are a hit and miss because the ones you ARE able to kill will have a fate-like essence surrounding them. That means they will revive. Some will never die and continue to revive and attack you. An example is the first city you get to in the plains where a side quest giver is collecting arm bands. Attack him and everyone else and you will see what I am talking about. Guards are also unlimited and always seem to match your level in some way. If you resist arrest it takes a very long time for them to cool down so to speak.