Good questions. Unfortunately I don't have comprehensive answers to everything. I'll try find out more, but for now I'll give the info/read that I do have.
1. Sorry I don't have any info on pre-order bonuses being potentially attainable later. I would assume not, though.
2. Unfortunately I haven't seen enough be sure, but given the nature of the game I'd be quite surprised if anything was missable.
3: No I don't believe so.
4: Outside of potential performance/graphical differences, the game should be the same. It supports cross-play, so game mechanics should be the same across all platforms.
5. PEGI 12 for people served by that rating system , T for Teen in areas that use ESRB ratings. If you're not in this region you should be able to see your local rating on the game's store page.