I can't answer all your questions but here goes:
Yes, I did this.... tried to do it -then it was SO cold that night (it was a freakishly cold night in Jan) that I had to leave before the show and they didn't sing either- they said it was too cold to bring out the guitar or something. LOL
So I did it again another night and this was a solo trip and I still had fun! I plan on doing this again with my children- and at almost 3, barely 6, and 9yrs old- I think they'll have a blast!
It's not sometimes they show a movie- it's always. They'll even tell you like 7 days in advance (sometimes more if you get a cooperative CM on the phone LOL) which shows on which nights. They have benches out with 2 places at the sides for bonfires which are set by them (they only did one of them when I went though- slow season

) They sell snacks there at a snack bar- including also smores kits which are cool. They come with graham crackers, hershey bars and a ziploc bag of marshmallows- plus 2 sticks (you can buy extra sticks very cheaply). It's enough for many smores- so one $5 kit is enough for a family!
Someone comes out and sings- I don't really think I saw Chip and Dale sing but they do come out after a few songs so maybe they are singing under there/singing along? Then Chip and Dale go around to the kids (or the kids come to them LOL) and then the movie is played once they leave- and everyone watches the movie and eats smores or just sits around the fire if it's cold/etc.
I fully expect that my kids will have a blast and that my youngest will probably conk out in the stroller..... unless she's interested in watching the entire movie. LOL
(one comment though- be sure you have lots of time to get there and plan on it taking awhile to get back to your resort also...... or wherever you are staying the night. Because even if you drive your own vehicle- you still have to take a bus inside there to get to the campground area and a bus back out to the front also..... and I not only took a LONG time getting a bus to the front and then a LONG time getting a bus to DTD so I could then take that bus to the resort- but I have read many others having this problem also. So my suggestion is go forewarned that it might take awhile- and maybe plan it on an evening when you are already doing something else at Ft Wilderness- there's a lot to do there! Hoop De Doo musical review/dinner, Trails End Buffet for dinner, I believe there is a petting zoo and a wagon ride of some sort- but I'm not sure of the times. Or perhaps you could take the boat over from MK and avoid the bus problem?)