I've gone Halloween night twice and it is definitely crowded but YMMV depending on what part of the park you are in. You could ask 10 people and get 10 different answers. Some areas will be less crowded then others.
Just walking around and trick or treating, its crowded. The trick or treat lines move very fast though.
If you go in with a game plan and priorities set, you should do just fine.
LTT and Tony's are open during the party and all the other sit down restaurants take ADRs up until 6:00p.
Yes, many adults dress up. I don't though. I just wear a Halloween shirt.