My friend, "had" a MIL just like that. IN fact, her Husband, his siblings, and ALL the neighbors cannot stand her either. She is a toxic case.
That said: her husband and she, DISOWNED MIL. Period.
She was damaging them AND their 2 kids.
They disowned her. That was the only way.
Her Husband, had no problem with that, with his Mom. He KNOWS she is a toxic mess... and very very damaging... to him/his wife/their kids.
The woman... is beyond therapy.
THEY decide, what they do, how/when they see her if at all... and they spend all their special occasions and holidays, the way they want.... as a family. THEIR nuclear family.
My friend's MIL...even outright tried to make she and her Husband divorce. Blatantly making trouble for it to happen. And, messing with her kids emotions/minds.... and damaging them with toxic psychological/emotional black-mail and passive aggressiveness and talking "stink" about the parents to her Grandkids.
The thing is... she will not change. She does not have to. She has many people/family/relatives under her spell... ie: her control.
Trying to Psychoanalyze her... will be endless.. and will NOT lead to answers. Still.
Next: You DO need to know, and I'm sure you do... that your MIL making your Son a B-Day party...without you/Husband... was a GREAT manipulation.... a manipulation, upon your SON. A child. This... is sick. On your MIL's part.
Not.... to be overlooked.
It was manipulating your son Will... and making you/Hubby be ostracized in front of him... blatantly. Because, you and Hubby were not there, nor invited and everything, was done secretly. Even your Ex Husband... did not tell you. Probably because he was told not to... and so he was "manipulated" too... under the guise of him seeing his Son... invited by MIL... and told not to tell....
All the best,
Susan