Hs she even been near a bank? Because bank managers don't really see people anymore. She would have spoken to a mortgage advisor or an account advisor surely? She isn't losing her mind, she is worried you have found yours.
And I think you know, op, that a mortgage advisor could not and wouldn't comment on the possibility and suitability of a massive mortgage to you to pay off her debt! Especially without you there, and no knowledge of your own financial situation. The bank won't loan her the money for a reason- she's too high a risk- and you already know she won't pay you! She might promise to leave it when she dies, but you need the money now, not in 40 years!
And of course, just because it could be done, doesn't mean it will be done! Just like it would be possible for us to buy the house next to my mil, it will never happen. Even if the bank manager came round himself:)
She has decided in her head that the money she and her late dh paid to educate your dh has been set off against the debt she owes to you two. Did she really never mention this before? That she begrudged the money spent as parents and expects it back? Or if not back, used as a tool forever to get her own way?
Have you asked your dh if she has ever mentioned that the cost of educating him and paying for his degree would have to be paid back one day? You may find it was loosely agreed that way for his university fees etc and he hasn't quite dared tell you.
Stop working out your family finances with her in mind or involved. Write off the 15k as you won't see that anytime soon. When she mentions it say your bank manager and financial advisor say you can't afford it and refuse to dicuss it.
Why don't you see how you can move to a bigger place? How impossible is it really? See someone and don't mention having to pay anything for her. Your dh could ask her what he would be left and ask for it now I suppose? Why would you have a painting on the wall when you could be in the garden? Get some copies made? Put them up instead.
Take your full maternity leave and don't sell your jewellery. Don't pay for her visits- no wonder she thinks you have money to burn! She's not the queen, let her put her hand in her pocket. You can get a national express bus from pretty much anywhere to London for a pound, so let her do that, because that's what she would be ok with YOU doing if you visited.
Sell that chest of drawers as well. Dont tell her, stop involving her in your money and financial decisions she thinks she has a say in how you spend it!!!
You can probably get a lovely one off eBay or freecycle if you look to replace it. That's why people give heirlooms, money for a rainy day and to make sure YOUR ds gets you for a bit longer all day. Don't forget to keep a note so your ds can pay it back one day!! Joke!
This is long but I see real trouble ahead if you have to change your life so your mil carries on like this. What hold does she have over you that she thinks you both will do it?