1) Enemies get 'locked' on whoever deals the first point of damage to them. After that xp and loot is rewarded to that person only. So if somebody is fighting a lot of mobs and you AoE you can technically steal their xp. Healing aggro can rip mobs from them, and if they are unlocked mob steal happens in case your companion attacks them. Most people release or rev by themself, but if you see somebody you can ress and buff em up. Gratitude of the act can vary.

2) I personally don't care if people want to watch cutscenes. I skip, but don't force others to do it. If you get grouped with that kind of individual, either tell them it's your first time and you'll watch everything(at the start of the FP) or ignore them and do your thing. Worst case scenario is that you overlevel the content and solo them without worrying about 'gotta go fast(and die faster)' folks.

3) Pretty much, need on mainstat only. Most low level flashpoints aren't adaptive armor so even if you like the appearance, you won't be able to equip it or it will give you less armor than it should. If you need something for companion, ask if you could 'Need for companion'. Some people will be 'sure' if the gear is not fine, others will want it for themself so greeding is a friendlier route.

4) In the next big patch(that should hit on the 28th) a designer feature is being introduced that will make it so you can have whatever appearance you'd like no matter the armor you're currently wearing. Before this hits there isn't a convinient way of swapping looks, since you'd have to remove all the stuff from previous shells and put them in the next set, and that cost a bunch of credits at endgame. Collections window is also terrible to navigate, as there's no search option. And it only counts for cartel market items, so any ingame drops you like(or legacy/reputation specific gear) won't eb there but have to be stored in cargo bays.