Lots of good advice here OP - except from barryfromclareisfit who is talking s@@te.
Re the milk issue. Our DS switched from formula milk to cows milk around a year old - then promptly started throwing up. GP and dietitian said he was lactose intolerant so we put him on soya milk. Which he hated unless it was on cereal but he'd never 'drink' the milk - big puddle left in the bowl. Soya yoghurts he hated. We did manage to get him on lactose free ice cream, cheese and 'butter'. Our GP and dietitian said don't worry about it, there's more calcium in leafy veg than dairy (or it's absorbed more) so we dropped offering him milk altogether for a year, gave him lactose free ice cream, hid lactose free 'butter', cheese and soya milk in his meals. So extra butter in mash with a sprinkle of grated cheese, a dollop of butter in sauces before they went on his plate, extra dollop on veg, lots of butter on toast, cheese sandwiches, cheese on bolognese, extra cheese on pizza, cheese on baked beans with butter underneath, we'd add beans or pulses to bolognese or stews as they contain calcium too etc. Luckily he'd always loved broccoli, peas, baked beans and would happily eat dried cereal which often has extra calcium in it as a supplement.
A year later we weaned him onto cows milk gradually on the GP's advice - just to test how his tolerance was doing. He was fine. BUT he still doesn't drink milk, doesn't like any ice cream now, occasionally eats a yoghurt, hates chocolate, hates cream. Won't touch the milk left in a cereal bowl. But he's as tall as me at 12 years old and is doing fine. His teeth have never needed a filling and apparently are 'perfect' according to the dentist.
Just look up calcium rich foods and choose those she's happy or safe to eat - add them to dishes like bolognese or pizza as toppings if you need to.
As for getting a break. Does your DH work at the weekend (I didn't read every message - sorry!) If not, can't he take her and the kids out for a morning or a day (preferably) whilst you put your feet up or do what you need to do? DH takes DS swimming every other weekend and has since he was little, then they go for a McDs for lunch. He also used to belong to a dad's group that did activities at the weekend for dads and kids only (to help dads bond with their kids). DH and DS loved it. Worth him checking if there's anything like that in the area and the soft play groups are his friend, remind him!