One alternative to consider is to try to rent a different combination of accommodations. There are very few GV's at any
DVC resort, and December is the most difficult time of the year to try to rent.
However, you could put together a combination of 1- and 2-bedrooms, or maybe studios, which would give you the lodgings you want. In addition to having much greater availability, that strategy would allow you to rent from several owners -- Diane is right, you're going to have a hard time finding someone with 500+ points to rent in the first place, to say nothing of giving those up during December.
The other advantage of several rooms is that if someone cancels, you only have to cancel a studio or one bedroom, etc. You don't go to WDW after months of planning with only 4 people in a GV for $7,000!
However, in any rental, it is critical that both you and the owner you rent from have a clear understanding of what happens if someone's plans change. Renting is nothing like a hotel reservation, and
you can't just cancel. Many owners make their rental non-cancelable and non-refundable, and there are numerous situations where an owner may be completely unable to help you if someone's plans change.