You have discovered one of the frustrating things about Disney's on-line reservation system. My guess is that some genius in management asked that they set up the system this way, in hopes of encouraging longer stays. In the meantime, people looking for sites that are actually open are told that none are available. To me, they end up losing in this situation.
So, for now, this is how you play the game: Book as many dates as you need to before or after what you are actually looking for. The deposit is always the same- one night's rental cost. Then, the next day, or any time up to three days before you arrive for that matter, you can call and cancel the days you don't need. No penaly. No problem.
Dumb, but that's the way it works.
TCD