Like figment said, you'll be missing fireworks if you leave at 9pm, and this the one of the main reasons I go to the Halloween Party every year. That being said, you can catch the Parade then leave maybe.. Photo ops with all the characters in their costumes is always a blast, watching the Cadaver Dans, getting buckets of candy.
I went on Halloween Night last year and it was like all the other nights of the Halloween Party. Since all of the parties normally sell out, and it's the same capacity at each night, I'm thinking it's the same. There are parts of the park that are less busy than others, and other parts that are a bit more busy, NOTHING like a normal day at the park though of course. Most of the rides themselves have no wait and all and you can walk right on.