The MHP ticket is a park hopper, so you'll be able to get into both DL and DCA starting three hours before the party official starts (so at 3:00 pm for a 6:00 pm party and 4:00 pm for a 7:00 pm party). You can hop back and forth all night if you'd like (as long as DCA is open), but all of the party-specific activities take place in DL.
I have never seen Roger Rabbit before. Last year we met with Donald & Daisey Duck, Mickey, Minnie and Pluto but as Figment said anything is possible.
Almost all of the MHP will sell out, but I've found the crowds to be generally pretty manageable. A sold out party isn't the same as a capacity closed park. While DLR has never said how many tickets they sell per night, it's not anywhere near the capacity level of the park. My guess is that it's similar to what you'd experience on a non-peak mid-week day. There will be lines, but they do dissipate as the evening goes on and you'll usually find the longest lines at the party-specific activities (e.g., characters, shows, etc.). Now, I have heard that crowds over in DCA get pretty high during the party hours because guests with hoppers head over there after DL closes. Also DCA closes fairly early (8:00 pm), so you won't have a lot of time over there anyway.
The lines for the Halloween Characters can be more of a challenge time wise then finding a good spot for the parade.
Sorry, a bit of confusion. In past years, there were three spots (left, center, and right when looking at the trains station from the flag pole), which all feed from the same line (guests lined up and meet all three villains - one at a time). Last year, they changed it and there were only two Villain meet and greet locations, one on the left and one on the right of the train station stairs (see the red back drops in the photo below). Two villains met guests at the same time; one at each station (e.g., Gaston was on the left and the Evil Queen was on the right). Each location had its own line. So what I was meaning was that if Maleficent does come out, you'll need to know if she's going to the spot on the left or the right because you want to make sure you're in the correct line. (Now all that being said, it's possible that they'll change the set-up again this year.)
Bring on the pumpkin products! 
