what time is the earliest you can check in both at WDW and Universal Resorts?
From my understanding you can check in whenever, but they may not have your room ready.
DVC resorts are 4:00 pm (to have the room) but again, you can check in, and have them hold your baggage (even if it's in need of refrigeration, they'll do that for you) at any time.
Not familiar with US/IOA policies.
likewise, zero experience with us/ioa I'm afraid
I know at the Grand Floridian, they gave me the room key and everything else I needed but just wouldn't tell me the room number until it was available