In the past, the restaurants have required a ticket for any ADR starting roughly 15 minutes BEFORE the party starts. This is to prevent people from taking in the party atmosphere without paying the extra for it. Now, if you're in a meal and your ADR is "fine" (that is, starts before that cutoff time), then when you exit you will be escorted. I have just never heard of anyone being escorted into a restaurant past party time.
In regards to CP, I'd err on the side of caution here, since it DOES require a $10pp hold and if you miss it (due to not being able to get in during the party), you will be charged that. The dining line CMs are notorious for poor and incorrect information regarding day to day park operations, so that's another bit that would cause me to mistrust what they have said.
I'd keep looking for an earlier one on that party night for sure, and see if you can shift schedules around to place it on a non-party night if need be. I, personally, wouldn't even think of trying to keep an in-park ADR after party start time without the ticket.