As far as I know, CRT, the Plaza and Tony's will take late dinner ressies (up to 5:50 p.m. or so). Many guests will try to get a late PS for those restaurants so that they can stay in the MK between the 6 p.m. closing and start of the party. LTT will be open during the party so you can get later ressies there if you like. You can call for a PS there now - LTT is being promoted as an add-on to
MNSSHP tickets and as the only restaurant that will be open during the party. They are doing the same thing for
MVMCP.
If this year's procedures are the same as last year's, you will not be able to enter the MK to get to your PS at Tony's, the Plaza or CRT with just your MNSSHP ticket. You'll have to use a day of admission to enter the MK before the party starts. If you want to be absolutely sure you can enter the MK with just your MNSSHP ticktet, you will have to reserve the add on dinner package at LTT. Again, you can call and do that right now - just ask for the add on dinner package. If you choose this, try for a PS around 6 p.m. You'll be able to eat during the time between closing and start of the party. You have to wait for the 60 day mark to make a PS for the other MK restaurants.
That said, last year for MVMCP, guests were allowed into the MK as early as 4 p.m. with just their MVMCP ticket - no "regular" day of admission ticket was required, even though the party didn't start until 7:00 p.m. Those guests were given wristbands and allowed to stay in the park while the day visitors were leaving.
No one knows for sure how this year is going to work for either MNSSHP or MVMCP (that is, how early you can get in with just the party ticket). Many MVMCP guests were upset last year because they thought the parties were too crowded and assumed that it was because people without MVMCP tickets stayed in the park for the party. Apparently, Disney stopped giving out wristbands shortly after 7:00 p.m., when the party officially began, so I'm not sure if the assumption was correct.
Anyway, the real answer to your question is "No one really knows when/how early Disney is going to let people in for the party this year". I'm hoping that those who go to the parties early in the month will post and let the rest of us know.
Good luck! Hope this post wasn't too long and actually answered your question!