Are your
MNSSHP tickets linked to your profiles in MDE? It seems that people who have park AND MNSSHP tickets have been able to book FP for all park days PLUS FP during the 4-7:00 window. I didn't do this myself, because I am going to MK on my MNSSHP day, so I can book FP that day anyway, but from what I understand you can book 2-3 FP, but the starting and ending times for those FP MUST start at 4:00 or later and end before 7:00. So most people are just doing two FP from say 4:30-5:30 and then 5:30-6:30 approximately.
I realize this isn't advertised, but in the MNSSHP threads, it is heavily discussed and Disney is well aware of the fact that people are doing it. The problem has been that if you have FP outside of the 4-7 window, it is triggering Disney to delete some FP for those people's trips.
I also don't know that it will allow you to book the party FP if you already have 3 FP at another park earlier the same day. Most people I've seen aren't going to a park the day of their MNSSHP and are booking the 2 FP during the 4-7 window.
If this post isn't allowed, please delete it, but Disney is clearly aware of people doing this and allow it, even if it isn't advertised as a benefit of the party ticket.