One quick aside:

As hard as you and DH are trying to explain to MIL "I don't like this" she's trying just as hard to explain to you she's entitled to do it and she's angry at you for not allowing her way.

It doesn't matter how many pages you write, how nice you explain it or how easy to digest it is...you aren't on the same page or speaking the same language. If you want change, you basically have to dog train, which is classical conditioning. Extremely small steps, with lots of rewards. Check out actual dog training... You literally do the smallest and shittiest steps and they get a treat. You've just got to figure out what that is for your MIL if your DH actually wants to "train" her to not be an entitled asshat. Cuz, she's trying just as hard to "train" him to accept her abuse. She's been this way for what... 60 years? One 4 page letter isn't going to change 60 years of habits...otherwise quitting smoking wouldn't suck as much as it does.

My suggestion is that if your done (and it sounds like you both are) is to be done. Don't send the letter. Cut contact. It'll protect you from the extinction burst that's gonna follow. It doesn't matter how many letters you send, it doesn't stop the flying monkeys or the guilt becuase abusive parents lie.

My MIL and FIL are extremely racist. They want all family events to be racially segregated and then threatened to hurt me. They openly state to this day they can't figure out what they did wrong or why their son doesn't speak to them. They know that we think they are racist too and openly write to us that racism doesn't exist, and his mom gave birth to a white child so they can't be "a racist". That makes no fucking sense... But you know what makes less sense? When I forward this on to FMs they still whine about "poor MIL" and how she's just "hurting". No matter what you say you can't win so just save your breath.