Theoriticall, you can only use 485 based
EAD after coming back from oversee. Otherwise, your 485 will be considered as abandoned if being caught.
2) According to regulation, J visa without a waiver is not allowed to file 485. There are cases in which CIS officer ignores this requirement. Chance is that you may be caught when you come back on AP and cancel your 485, though the risk is not great.