Well the rooms not declared are continually being sold through the Cash side of the business (as they aren't apart of the Condo Association yet). Disney has been selling those rooms for cash for a while now and it is a busy time for Disney Cash bookings. So I wouldn't count on something 1 month out especially for it being the size and time of interest, but possible. Also I know
DVC doesn't pull all available rooms from cash, when I was buying there was some I know for a fact they wouldn't pull from their inventory.
Though I would simply, as suggested by others, make it a condition of sale for the OP. If I was the OP I know they do extended deposit payments and from my understanding the points can be used to book vacations after your estimated closing dates at first deposit. Also one thing to ask is I thought all reservations had to take place after your estimated closing date so that could be a hamper to things too depending on when Disney allows the OP to close.