The week between Christmas and New Year's is the absolute busiest week of the year at WDW, and so you need to plan accordingly. MK will close at capacity at some point that week, and it's possible MGM will as well. And even in the parks that aren't closed, you'll feel as though they're bursting at the seams.
All this really means is that you have to plan more carefully, be sure to have PS's for any times that you know you'll need to eat (this is not the week to rely on counter-service food, since those lines will be very long as well), use FastPass, and know how long you'll stand in line for something before you get in line.
If MK closes in the morning, it will likely re-open after the parade, so if you really want to go to MK on a given day, just wait until the afternoon and chances are you'll be fine.
And do maintain a sense of humor. Remember, you're in the busiest vacation destination in the world during the busiest week of the year, and you chose to do that, for whatever reasons.
Just enjoy it!
