Terry,
Thanks for asking your question to planDisney!
As long as you are entering a park during their posted operating hours, if you have a valid
Theme Park Reservation you are guaranteed entry. Guests that park hop to the Magic Kingdom would not be taking your place, so no worries there!
Admittedly you are arriving at a busy time of year, and certainly there is the chance that the park will reach full capacity on New Year's Eve. But it is those guests that are park hopping that run the risk of not getting in, since it is such a popular day to be in the
Magic Kingdom Park.
BONUS TIPCrowd levels on such days can make certain activities take longer than others. I was there for the 50th Anniversary of the Magic Kingdom and faced similar crowds. Make sure you use their
Mobile Online Ordering system in the
My Disney Experience App so that you can get your meal with less time in a line. But even with this system, don't wait until the time you want to eat to place the order. Order pick-up times will book up, and you need to get the order in early to eat at your planned time. For example, if you want to eat at 12:30 pm, put your order in around 10:00 am. You get to choose from available order pick up times, and if you place your order that far in advance the 12:30 pm slot may still be available. If you were to wait until 12:15 pm to place your order, the next available pick-up time may not be until 1:30 pm or 2:00 pm.
Have a Magical Vacation!
Doug