What you don't seem to be appreciating is that the party is only from 7pm, so if you only have the party tickets then strictly speaking that is the admission time, the 4-7pm is a bonus as you haven't paid for a regular park ticket. Like it or not this is only fair, after all if non ticket holders were allowed to stay in that bonus three hours and see the parades and fireworks and so on I certainly wouldn't be happy about it, if you see what I mean. Besides, you are not disallowed from going on the attractions for those three hours, you are just not able to have the perk of the FP+
Just adding to clarify, you only have a three day PH which you plan to use on other days I presume (4 days in all) so that is why they want you to cancel one of the other days. No FP+ are able to be scheduled during EMH or special events like parties, they never have been even before the days of MB,
MDE and FP+