KC, good on you for not taking your in-law's BS lying down. I can't believe she would open her son's mail, much less yours.
I think you're right to cut contact if she's the type to get involved with something that's none of her business. Families are often threatened when the partner quits because they know full well they'll have to deal with the A more.
Glad you're feeling much more peaceful. Going no contact will help with that.
"I was looking for the key for years....but the door was always open."