I don't understand the legality of not abiding by the Will.
The fact it was done 20 years ago is not relevant.
It was his father's wish they should have some money.
Did you do this through the solicitor?
Had probate occurred then so you all knew how much money was around?
Not abiding by a will unless it's done legally is illegal.
(Are you in the UK as noting the timing of your post)?
Your MIL is relatively young- 7 years older than me. In today's terms she is not an 'old' woman! she has at least ten years of good health left I'd say when she might want to live a bit and enjoy life. You are talking of her as if she is in her 90s!
She may live for another 20 years, so £200K is not a lot of money in terms of assets for someone her age.
It's an average of £10K pa on top of a reduced pension and any state pension she has. It will be eaten away by inflation over the years.
If she decides to give your DH any early inheritance that's her choice. BUT as a family you were all wrong to alter the Will!
As before, how was this allowed? It might have been a family decision but it wasn't legal and it wasn't your place to suggest it even though you had good intentions.
You acted prematurely and now that the full extent of her finances are known, it's come back and bitten you on the bum.
You ought to have planned your OWN finances better with some contingency for loss of earnings, which you didn't, and that's why you have to move.
Sorry but you sound very controlling. Aas a DIL it wasn't your place to suggest your FILs will was changed. it's not your role now to suggest your MIL coughs up to help you out.
She may well have done that but she hasn't and there is sweet FA you can do.