On-site guests get first dibs in the sence that both on-site and off-site guests can begin making ADR's 180 days before their trip, however, on-site guests can make reservations for the next 10 days of their vacation as well. Off-site guests can only book the 180th day from a given point so you would have to return every day to make an ADR for the next day of your trip.
There are only a couple of places that you need a credit card to hold (i.e. California Grill, CRT). Dinner shows have to be paid in full at the time of the reservation.
You should cancel any ADR's you will not be using just to be fair to everyone else, but in reality the only ADR's you need to cancel and not be penalized for are any ADR's that you need a credit card to hold and dinner shows.
I think that about covers everything.