Greetings, David.
Yes, you can book a Disney resort room for just one night and get tickets separately... you just can't buy a Dreammaker package, because it has a minimum of 3 nights...
We've always had reservations before arriving. I guess the chances of finding available rooms on arrival without ressies depends on the time of year you arrive, and what level hotel you want to stay in. I can't imagine that there wouldn't be a room ANYWHERE in WDW, but again, it depends on the time of year.
Someone else here will know for sure, but I think you can make reservations for room only and cancel at least 5 days out without being charged a cancellation penalty. To be safe, maybe you should go ahead and book a room!
Kim
