I can't answer all of them but hopefully I can answer a few; keep in mind however that my own GradNite was 6 years ago this year (jeez how the time flies!)
It will be extremely crowded. I remember going and it was wall-to-wall kids pretty much everywhere, especially in the areas with the more popular rides (SM, Indy, BTMRR) and in the areas where the "dance clubs" are set up. As someone who only went during off-season times before that, I was completely shocked and not at all happy cause I wanted to ride the rides. silly me

I remember talking with my friend that I was going around with that Disney must have decided to pack as many kids in as the Fire Marshall would allow so they could get as much money as possible.
If I remember correctly, they are not running during this time which makes the lines for the aforementioned rides horrendously long. Our school got there pretty much at the beginning of the evening and my friend and I went to Tomorrowland as soon as we could but there was already a 2 hour wait time and it just got longer as the night progressed. Even Jungle Cruise had a 45 minute wait time. IIRC the only ones that we really walked on were the Fantasyland rides (Snow White, Pinocchio, Mr. Toad's).
IIRC there are certain places open for food during the night but not every place is open (i.e. Blue Bayou, etc.) I think more of the popular quick-service places (Bengal BBQ, Hungry Bear, etc.) are the ones that are open.
I hope this helps. Just be prepared for large crowds and as the night winds down, more and more kids are going to be finding spots on Main Street to crash. I think that is what surprised me the most was the amount of kids that had just found a spot against a wall or if they were lucky, at a table with chairs, to put their head down and sleep. Not sure why I was surprised though

as my school made us go to school all day and we left almost immediately after school was over (no time to sleep). I'm sure every other school is like that too; don't want to lose out on all of that money that is gained while kids are in the seats!