Oooo, tough call. We have gone for Christmas through New Years Eve for a few years now. Each time, we decided to avoid MK during the day and just managed to slip in right around 10pm. (Although, I would recommend it…or chance it) The park reached capacity at 11am on New Years Eve - we just happened to get lucky. My DH suggests trying to maybe take a boat in from WL or FW to be with the resort folks. ??? Good luck!
One other suggestion is to hit the early version of the New Years Eve countdown fireworks. Look for or ask about the pre-New Year fireworks schedule. They usually happen a day or two before. Less crowded. Again, best wishes! (Maybe, we’ll run into one another - tee hee!)