These are good answers.
I don't think Disney holds any reservations/capacity back. This timeframe in question is one of the highest demanded times in general (rates reflect that). It's my position that 99% of FW sites for this time of year are booked very close to the 500d/499n max window.
A portion of the Fort guest profile includes locals who book weekends WELL in advance and come in a couple times a year. We have a thread currently re: Tent Camping and the OP related how they were in 1500 and it was quiet Sunday into Friday and then BOOM! the loop behind them which had been empty (a Full - 1600?) exploded with people and noise all weekend long.
See Post #1 I think the Full guests who ARE there midweek are offered sites in 2100 and the weekenders are put in 1600-1900.
A good number of the local/weekenders probably have AP's and those that don't socialize at the Fort. It appears that families and/or friends and/or neighbors make up a large part of the 2-3 night weekend influx guests who are there to socialize and party. They're not isolated, unfamiliar strangers camping around each other who sort of mind their own business and go around and do their own thing. And they want to see all the Christmas decorations at the sites that are usually still up during that time.
The other WDW resorts don't have that kind of frequently returning guests like the Fort does. Heck some hotels are still closed on property. But folks will come to the Fort even without park tix (like you are considering). Also you don't hear much of frequent AP holders who book a hotel room on property for their park visits or on property hotel stays with no park days - but FW guests do.
This year I think people sit on their reservations and then decide closer to their arrival date whether they still want to undertake the vacation given the cutbacks, financial hit, and when they might ever return (more so than in a normal year). I agree that cancellations will come up perhaps more frequently and can be snagged closer to time so if you have the flexibility to wait, there could still be an opening.
I hope you find your dates - so keep trying.
Bama Ed