You can get PS for the Liberty Tree right now; just tell them you have tickets for
MNSSHP that night.
But you may have to be persistent: I called to get PS for the LTT on October 19th, and it took four calls before I was successful. The first three times, I was told that Oct 19th was totally booked. On my 4th call the next morning, there were lots of time slots to choose from.
It's a character meal; usually it's Mickey, Minnie, Chip & Dale, Goofy and Pluto. They're dressed in their Halloween costumes. If you've never eaten there, it's served family-style at your table, and is really good.
I think the price runs about $23 per person or so.
If you get PS somewhere between 5:15 to 5:45, that should put you in just about the right timeframe to be able to finish just as the party starts. Also, one of the photo booths is just outside this restaurant, so you can walk right out and get into the line for photos early, before the line gets too long.