I don't think they can close it until that reservation ends - well, I guess they could close it, but I don't think Disney would allow the contract to be transferred in their system while there is a pending reservation.
As long as the contract you are signing states the correct closing date, I don't think I'd worry about it too much. If your reservation is pretty far out, maybe just wait until it gets a little closer? I don't think I'd wait until the reservation ends because I'd want the contract to close as soon as possible after that to get my money.
I was the buyer on a contract last summer that had a delayed close. The seller and I both sent our closing docs in prior to when the seller's reservation ended. I did not give my money to the title company until we neared the date and I confirmed that the seller had already signed their documents. Then, the sale was promptly closed after that. Seemed like a very normal process.
