I booked FPP for MNSSHP with my regular MDE account. It’s not so much the order of the party day in your plans, but the order in which you book your FPP. Book your FPP for regular ticket days first (e.g. trip days 2-7) then FPP for your party day last (e.g. trip day 1). Then watch your account. I think the problems aren’t so much with FPP for the party being deleted but with FPP for park days after the party.
My FPP for my last trip day (day after MNSSHP) were deleted at some point but all I did was call guest services, they could see all my tickets including party ticket to verify I could book FPP for all those days, they could see what FPP were deleted, and they gave me new FPP for the same attractions I had previously booked that had been deleted (and thinking back on it now, I think they were anytime FPP because they showed up without a set time slot and I’m pretty sure we used them at different times than my original plan).
You could maybe do the dummy account thing (I will defer to the experts on that as I don’t know how that works) but it probably wouldn’t be linked to your resort reservation if you have one.