I would keep well away from his mother as well as his enabler of a father. You also need to raise your boundaries a lot higher as well, you cannot respond to people who are inherently unreasonable (she certainly meant to be both rude and chastising) and you need to call her on such behaviour each and every time. Bad behaviour should not in any way be at all tolerated.
I would also read up on narcissism and see how much of that relates to your future MIL.
What does your man think of his parents these days; does he really think a letter is gong to make any difference (you are right in that it will not make any difference). Is he himself in a fear, obligation and guilt state when it comes to them, does he still seek their approval?.
It is not your fault she is like this, you did not make her this way. Her own family of origin did that lot of damage to her. Was not all that surprised to read that FIL (himself a weak bystander of a man) enables her, women like this always but always need a willing enabler to help them.
With regards to yourself raise your own boundaries a lot higher and do not JADE in future (that is what you tried to do before and it does not work):-
If I try to justify my decision, the toxic would probably insist that I was wrong, because I didn’t do things their way.
If I try to argue with them, the toxic would probably take this as a personal attack against them, and escalate the situation.
A toxic person is so self-focused, they can’t accept any point of view that doesn’t 1000000000000000% agree with their own—and they generally take any disagreement as a personal attack.
If I try to defend my position, the toxic would also not be happy about this, since I would be refusing their control over me.
And trying to explain my choice could be the worst choice on my part, since the more information we give a toxic, the more ammunition they can use against us.
Do read "Toxic Inlaws" by Susan Forward.