If you're going to be at her place for a while, can you prep and transport the potatoes and the sauce separately, then mix together and bake after you get there? If you have time before dinner and space in the oven, that should keep it from separating. Reheat the sauce, whisking it to make it all homogenous again, then add the potatoes or pour it over the potatoes, or whatever. Put it in the oven. Throw some extra cheese on top toward the end of baking for its beautifying effects. If scalloped potatoes were a must, I guess this is what I'd try. It's hard to keep everything from separating when it needs to be reheated. Maybe Barry or someone will have a better idea.
My sister is fixing the majority of the dinner at my mother's house this weekend and she assigned me a side dish and dessert. Every side dish I suggested she said her kids/husband didn't care for. She said something about a squash casserole would be good. Squash casserole? Never made one and it would probably have the same issue as your scalloped potatoes- the sauce would likely separate on re-heating. I think I'll do roasted broccoli with lots of lemon and parmesan on top- wash and cut up the broccoli here and then it just takes 20 minutes or so in the oven (while the roast rests you know). Desserts? My husband wants chocolate, chocolate, chocolate- cheesecake, pots de creme, those little melty individual chocolate cakes. He's the only one who wants that, though, so I'm baking some cookies and brownies and gingerbread and my mother plans to pull something dessert-like of indeterminate age from the depths of her freezer. I'm sure there will be enough variety for everyone to find something they'll eat.
Spanakopita sounds excellent! If it's too much for you all, how fast can you get it to Maryland?