I provide meals for my 86-year old MIL with Alzheimer's and her son that lives with her. I need options when I'm busy or burned-out. She's no longer able to cook as she was burning everything. Her son (who is basically a mooch but his living with her keeps her in her home) is responsible for fixing her a simple breakfast and lunch (from groceries I buy) but I've been cooking the suppers and sending them over in divided containers, ready to microwave. I called a local all-you-can-eat American-style buffet and they charge $5.99 a pound for take-out, and I'm thinking I may go that route for ease sometimes.
I am hoping that your mother-in-law or her live-in son are reimbursing you for the cost of groceries or take-outs. Groceries for two extra people isn't cheap.
As to frozen foods, Lean Cuisine generally doesn't have preservatives. I don't know about the sodium content though.
Anyhoo, when you are absolutely burnt out (been there, done that) portion out Dinty Moore hearty beef stew or cook a few egg noodles and top with Swanson's Chicken a la King. Both come canned. Tuna or chicken salad sandwiches are also easy. Starkist even makes an already prepared tuna salad in a foil pouch. Add some grapes or berries to the container and instant dinner. My mom loves the microwave White Castles burgers from the frozen section. Hormel canned chili is another favorite. Add some chopped onion and cheddar and it's actually pretty tasty. I make her fresh coleslaw twice a week which she eats every evening along with three prunes and an Activia yogurt so it's not all junk. :)
It's really difficult this time of year when my husband and I just want to grill a pork chop or something and have corn on the cob and a salad. Mom's teeth aren't so great so that doesn't translate at all into a microwave meal. I am so sick of making casseroles. The one I don't mind is Rotel's King Ranch Casserole. It's on their website. It's easily adaptable to whatever ingredients you have on hand and freezes well. Good luck! I truly understand your frustration.