You did have a choice. And IMHO your DH was v unreasonable to ask you. MIL doesn’t need care and, given her youngish age, will likely be in your household for your DC’s entire childhood/teenage years. That’ll affect the whole family dynamic, which isn’t great if you don’t enjoy her company.
What are you working so hard for if you can’t relax in your family home? It’s surely better to live as a family in a smaller, less convenient home. She’s been v unreasonable giving £20k to other family members then expecting to live with you.
It’s not great to share a house with relatives, without seperate spaces. She must also feel v uncomfortable not having her own space. A kitchen is the hardest thing to share if you’ve been used to your own.
Can you have some work done to give her a seperate space, effectively her own set of rooms within the house, including her own kitchen, e.g, could the spare reception room have a kitchenette fitted to become her kitchen, diner, living room?
If not, and you can’t immediately buy her out, then you’ll have to sit down with your DH and set some ground rules, e.g. MIL doesn’t say racist things in front of you and the kids, leaves you alone after work, only reprimands the kids if neither of you are there etc. Your DH needs to speak to her about it.
If you buy her out, she’ll have to get herself a flat. If she knows that’s your and DH’s preference, she may choose to give you more space to retain a larger home.