1) Alice and Ilias keep everything, but when you choose the other one they are unavailable until you start a route that has them.Thanks.
A few more questions, if you don't mind answering of course.
Do Alice and Ilias keep their Levels/Job levels/race levels between different NG+ playthroughs?
Are there any guides I could refer to for companion recruitments and finding job/race switching items for part 3?
Or, at the very least, are there any non-overworld companions I should try and not miss in part 3? (I'm not especially concerned about spoilers)
Do the side quests I completed in previous playthroughs appear in NG+, and if they do, is there any detriment to not completing them in the said new playthrough?
I have heard of the collab event, is there a way to avoid playing through it (I would guess that it's optional in the first place)? I would like to play through it only once i'm on my 3rd run.
What are the requirements for Chaos route? Is it just to go through the game and get both the judgement and worldbreaker endings?
2) I don't know of any complete guide to part 3 yet but I wouldn't be surprised if one exists.
3) Side quests you complete can be done again in New Game+, but you do keep the rewards from your previous playthrough so mostly there isn't a big need to. I believe the exception is the 4 bandits sidequest that spans the whole game - not that it is relevant to the final route, exactly.
4) The collab events are two completely optional side quests. In fact, I'm not sure you even *can* do them on a part 3 save since they seem to have disappeared but I also haven't fully completed the game.
5) You simply need to complete both the other routes.