Most lines are not bad but there will be longer lines for the rides that are very popular like 7DMT. Also if you want to meet specialty characters like Jack & Sally or the 7 Dwarfs or Jack Sparrow - they have hour+ lines.
There are no accommodations at the party. I think the assumption is this is something extra we chose to pay for to enjoy the special parade, fireworks, trick or trick, dance parties, new characters and the attractions are there to ride with shorter than normal lines. We do just fine at the party and if a line is too long for us then it is something we wait and do on a regular MK day. With all the party stuff there isn't enough time to ride everything anyway.
Note you can enter the park at 4 pm with just a party ticket, no day ticket needed. During those three hours you can get 3 FP+ with your party ticket and use your
DAS. This could allow you to get about 4-5 rides done.