Ordering the night before helps them to modify the ingredients where possible so that you can enjoy the same dish whether appetizer or main. Desserts are more limited but I have had them make an apple crisp as a special request. If you don't order before, don't worry as they can still often offer at least one dish without the sauce etc. They will bring gluten free bread if you want in the dining room and the buffet will have it for breakfast, but sometimes tricky to locate it. On boarding days for lunch, I can have the salmon and at night steak is safe.
I agree that the buffet is trickier but you can ask for the chef to come out if the staff don't know, but I normally stick to salads and the carving station for lunch and always eat dinner in the main dining.