Yes, it will be crowded, especially as you get closer to Christmas day. And yes, do MK on the 18th and 19th. Whatever you do, don't do MK on Saturday. Thursday is Early Entry day. Go at 8 a.m., or whenever the EE time is, on Thursday and then head out later in the day to another park. If you don't qualify for EE or can't make it at the opening, then I would skip MK on that day. Friday should be less crowded though because so many people will have gone the day before. Again get there at opening. It might work out pretty well because the park should be running with a full staff and on the holiday hours, but hopefully not ALL the Christmas travelers will be there yet. I like the Day After approach to the parks. Go to a park the Day After it has had its Early Entry. We did this on our Fourth of July trip, and it worked great.
Your schedule will depend on if you qualify for Early Entry. If you don't, I would say go to MGM on Thursday, MK on Friday, AK and Epcot on Saturday (I'm not a big AK fan, so I can get it done in a few hours, plus Epcot is the biggest park and seems to absorb the big crowds better), MGM again on Sunday, MK in the a.m. on Monday followed by Epcot. On the last day, go back and do the things you have missed or your favorites. Is the Mickey Christmas Party held while you are there? Is that a possiblity? You also might want to go between MGM and Epcot on some days instead since they are so close to one another.
If you are staying on site, I would do Early Entry and then hop to another park around lunch time, preferably the park that had EE the previous day. EE at MK is on Thursday and Sunday, and at MGM on Tuesday and Saturday. You might want to do EE Wednesday at Epcot if you want to go on Test Track or Mission:Space.
Whatever days you decide to do the parks, GO EARLY. We made PS's for breakfast in the parks on non-EE days before the parks opened. That way we got in Early and NOBODY else did, except the others at the restaurant.
Have a great time.