Hi asmisp,
In answer to your questions:
1) Yes, the 'appointment' program is updated when 'slots' become available, so you might find it useful to check back on a regular basis.
2) Although there might be some exceptions, the individual centers do NOT do any of the scheduling themselves, so there wouldn't be any 'local waiting list' to put your name on.
3) I don't think that GMAC has published the type of cancellation/reschedule data that you're looking for - but even if it was available, that data wouldn't really mean anything in your situation. Either a spot will open up or it won't (past appointments from other Test Takers wouldn't impact whatever's going on this coming April 2nd).
A 30-minute travel time to a testing location isn't bad. How much much closer is the DC Testing Center to you?
GMAT assassins aren't born, they're made,
Rich