First off - You can go on the Disney website now and look for ADRs for the dates you're considering, and get a feel for what's available. In fact, go on and look at dates for the next week or so, and you'll see how many places still have "last minute" availability. And as a PP said, people cancel reservations all the time. If there's one thing I've learned on the Dis, it's that if you're looking for an elusive ADR or FP+, don't give up, just keep looking, and you might get lucky!
I bet you'll be able to get plenty of good FP+, too, if you're a party of just one or two - and again, if there's one you want that's not available, keep looking.