Oh, okay, thanks!!Yeah, I battled with every trainer with the EXP share on and I was 10 levels higher than the Elite Four.
No it doesn't, you can go back anytime you want. All rooms are still accessible (some scientists and grunts are even still there) so the Master Ball and any other items you didn't pick up should be there. ;)
Ill probably use it on deoxys since a lot of people are saying that rayquaza is easy to catch