Oh Cordelia, I read your mother's letter, and even her tone of voice sounds exactly like my mother's. It could have been written by mine, it really, really could. You have my sympathy.
Many years ago, I had to do what you are doing now. I too felt ill.
Mine wrote letters too. They were designed to try and sound reasonable and nice, but the undercurrent was always there. At first any communication from her made me feel sick and ill, and sort of ....hunted.
I was in my 30s, when I called it a day. She had just told me (among other things) that I wasn't the daughter she'd wanted (and this because I hadn't rung her one day to enquire about her blood pressure). That my brothers were better people than me. They cared about her more than I did. They knew how to behave etc. Oh, I could go on and on about the awful things she said...
But it made me ill, and it affected my ability to be a good mother, so I stopped it.
I am now in my late forties. My children are young adults, and I don't regret doing what I did, because I saved myself and I saved my daughters from being infected with my mother's toxic rubbish.
Nowadays, my mother and I only have an email relationship. And it suits us both fine. We send each other cards. Maybe three or four emails a year. (I think we've reached the stage where she feels lucky to be allowed to have any kind of communication with me, but we are talking about fifteen years later!!!!)
My brothers are still very much in contact with her and indulge her beyond all belief as they believe they are doing the right thing. And she abuses them. And she abuses their wives. And their children. 
As far as the letter goes, I would send a reply that gives her no satisfaction, but draws a line for you. I think I wrote the odd one to mine that said along the lines of " Thanks for your letter. I will think about what you've said, but in the meantime, let's just leave things as they are."
Sorry to ramble, but your post struck such a cord with me. It is so hideous to be on the other end of this suff from your own mother. And although what I have with mine is far from normal, it is fine. And i have a lovely relationship with my daughters. 