We've survived Christmas! We first visited during Christmas in 2009 and knew NOTHING. We had been to Disney in July and fought the crowds, but never anything like this. BUT, we loved it so much that we went back again in 2010. After skipping in 2011 (we went to a wedding in Disney in 2012,) we returned in 2012, 2013, 2014, and will go again this year!
Your plan is not crazy - in fact, it's very similar to my family's plan. You can expect MK to open at 8am, with EMH from 7am to 8am. Every year we say that we're going to get there for morning EMH. While we certainly try, we usually get there for 7:40am and enjoy the last 20 minutes of EMH. When we arrive around 7:40am, Space Mountain is around a 20-minute wait. We jump on there and by the time we're done, the park is open to the public. Our FP+ for Christmas are different than the rides we'd normally target. My mom doesn't do coasters, and we like to be together on Xmas day and experience the attractions together. So, we'll usually do Space Mountain during morning EMH, and book FP+ for rides like Buzz Lightyear and Haunted Mansion or Pirates. My sister and i LOVE Thunder Mountain, so we always book a FP+ for that.
I'd recommend getting to MK for morning EMH. I would target rides like 7DMT, Thunder Mountain and maybe Buzz for FP+. I never book Splash Mountain because the weather is way too inconsistent. It can either be 50 degrees or 80!
So you'll enter the park around 7:30am. You'll get in one big ride (like Space Mountain. When we entered last year, 7DMT was already a 90-minute wait!) After, the park will be open to the public. Let's say you have a FP+ for 10am. I'd use this time to either grab a quick breakfast (we do, you may grab one at the resort.) After we eat, it's around 8:30am. At this time, you can see the wait times starting to creep up (as shown above.) The Jungle Cruise (named Jingle Cruise during Xmas) is around 30 minutes. We usually choose to wait. Jingle Cruise falls under the "cult following" category for rides at MK. It's not a thrill ride or the first ride you go to at Disney, but it'll see a huge wait time as the day progresses - usually 60-65 minutes - so 30 is good. After we finish on Jingle Cruise, it'll be around 9:15. We have some time to kill before our first FP+, so we head to Small World. The sign said 20 minutes, but we walk right on. We're done by ~9:45am and walk over to Thunder Mountain. At 10am, we ride Thunder Mountain. We're done by 10:20am and head to Haunted Mansion. Last year, Haunted Mansion was a 5-minute wait when we walked over. It was too good to pass up. We rode Haunted Mansion, and then went to redeem our FP+ for Buzz. After Buzz, we had a FP+ for Pirates. Pirates wound up being closed last year and we were unable to exchange them given the crowds - but you catch the gist.
So yes, you can get to MK and if you plan things correctly, you can get in a good amount of rides. We'll usually leave around 1 or 2pm. Last year, we had a late dinner, so we rode the monorail to Poly and ate at Capt. Cook's.
As far as the crowds go, here's what you'll experience:
EMH: A decent amount of crowds. More than you'd expect for EMH, but not too bad. Manageable wait times.
Rope Drop: You'll see a solid pickup in crowds, but I remember our first Christmas thinking I was lied to. The crowds were NOT BAD at all, and the ride wait times are still manageable.
10am: This is when you start to see the crowds pick up. My theory is that most families either spend the morning opening gifts or having breakfast together.
11am: Now the crowds are really noticeable. Fantasyland is hard to navigate, Main Street is shoulder-to-shoulder
12pm: By now, the ride wait times are astronomical. Unless you have a FP+, there's no use in staying at the parks. You can catch the show at the front of the castle, but otherwise there's not much to do.
As you can see, your plan WILL work. It's tried and tested by my family and definitely doable. We always say that we'll go to the park after dinner but never do. We usually call it an "early" night and sleep in the next day. After traveling on Xmas Eve and waking up early on Xmas, we're pretty tired.
Check out my free guide at survivedisxmas.com. I have a lot of tips and tricks for surviving your Christmas vacation!