That's too many people, for too long a time, and too many dietary restrictions. You are very kind for attempting to accommodate and host everyone.
I wouldn't make breakfast except for one special brunch on the weekend, otherwise just have lots of options available. Oatmeal, waffles, cereals, yogurts, bagels. Trader Joe's has amazing croissants you can put out to rise the night before, they are really good!
For lunch have salad and sandwich ingredients available: nuts, cheeses, breads, spreads, salad greens
Let MIL cook for herself
I would maybe cook dinner once or twice a week at most, other than that I would really heavily on going out or doing take out.
Make sure to stock up on easy, frozen meals like pizza, and also staples like spaghetti.
Good luck!!