I would suggest that since plans have changed rather late in the game, and you are having this so close to a really big event, it should be simple, whatever it is.
Don't feel obligated to 'do it up big' just because you are doing it. I know it is 'the big day' and all, but really it is about the people and the time spent, so don't knock yourself out at the last minute here.
And if it is only a few folks coming, and they are not in the position to prepare their own meal, I'm sure just being included and have that time with you on the holiday will be so special for them.
Our Christmas meal was often a roast, when I was a kid. So it sounds really comforting and traditional to me. It can be a one pot type of meal, with little fuss, but delicious and satisfying as well.
Especially if the folks at Christmas dinner will be at your open house later in the weekend, you won't want to or need to make a fuss. Setting a really nice table I think can often make even the simplest meal seem like so much more, so fancy up the table and keep the work in the kitchen simple.
Enjoy all your celebrations!