If you get seated right away you can maybe make it. They bring the food out to you family style, there's no ordering process except for drinks. If you tell your server right off the bat you are in a hurry that will also help. Also ask for the check as soon as they bring you your main meal. That always seems to be what I wait for the most at WDW, getting the check.
However, if they are running late, and its quite likely since your reservation is later and you will be a victim of all the late shows and snafu's of the night, you may be seated late. So get there early, like a half hour early and get your name in for sure!
If dinner runs over or near to the time of Wishes, you can have a decent view of the fireworks right outside the restaurant. You miss a lot but you see quite a bit and the music is piped in back there. So if its already started or is about to, I wouldn't waste time trying to squeeze in over by the castle, just see it right outside the restaurant. I agree that its always better to see them (esp Holiday Wishes) in front of the castle. However, I think you'll miss more trying to get over there if you exit the restaurant close to or after they have started. Party nights are always packed for fireworks.
All that said, I must add, I would really advise you against doing this meal at this time on a party night. Because frankly, I'd rather eat someplace else and get a good spot for the holiday fireworks. Again, I love LTT, but for me, I love Wishes more! And also, I have to say, there is so much to do at
MVMCP, I would
never waste what could potentially be two hours total (wait time to be seated, time to eat, pay etc) eating. We always eat before 6:00 p.m., preferablly before 5:00 even on party nights. I really would rethink this one. And again, I like LTT very much but I just think with what the tickets cost, and how much there is to do, its a shame to spend one minute past 7:00 p.m eating anythiing but the free cookies and hot chocolate! We ate at 4:00 during the Christmas party two years ago and still missed some things that night.