My family did Christmas Eve at Seaworld last year. We arrived at rope drop and left at park close. We did one up-close tour in the morning and had Christmas reserved seating. There was no traffic getting there in the morning and leaving at night but my hotel was five minutes from the park. We didn't do any rides on that day because we wanted to focus on their Christmas festivities. Granted, we visit the park once or twice a year so skipping rides is not a big deal to us.
I don't think the crowd was bad at all. We moved around freely and never at any point I felt uncomfortable. And I had a double side-by-side stroller with me. There was never a line at restaurants, except at Mango Joes's right before Shamu Christmas show at the end of the night. Just to compare, we went to Magic Kingdom the day after on Christmas Day, arriving at rope drop. And we had to leave at noon because I couldn't handle the crowd. It was wall to wall people. There was no concert of personal space there.
I think you would be completely fine if you arrive at rope drop. Do your big rides then. It shouldn't take any more than a couple hours since Seaworld doesn't have that many rides. And you will have the rest of the afternoon for non-ride, non-Christmas stuff (regular shows, animal exhibits, etc). The Christmas event starts at 5pm. The four Christmas shows will keep you busy all evening till park close. I barely had time to spend at Christmas Market and Christmas Town. So this year I decide to do two evenings there and spread out the four shows over those two evenings, no reserved seating. This way we will have some time to enjoy their other Christmas offerings.