OK, so I've been in situations (not at Disney) where I've shortened my hotel stay. Sometimes for emergencies, sometimes because plans changed unexpectedly. If Disney won't let you shorten your stay ahead of time, why not just do it at the hotel when you're checking in? "Ms. Jowntown, I see you're staying with us for 4 nights?" "Actually, I'm only going to be staying 3 now." Of course, this only works for people who are shortening their stays on the tail end. I mean, there can't be an issue with room availability if on the day of checkin, they're assuming you're staying for 4 days and then you end up only needing 3.
I wouldn't recommend this as a strategy unless you've been denied over the phone, but it might give a ray of hope to some of you who feel like you are being forced into longer stays.