I couldn't not read it - I'm not saying you should, just that that is my personality.
If you don't read it, I wouldn't send it back - that in itself is a reaction. I would just ignore. Perhaps if (ha - when!) she brings it up you could say "oh, I didn't realise you meant you'd SENT it - gosh, must have been lost in the post". That'll annoy her!
I think you have to decide if you want her in your life, and on what terms, separate to this decision about the letter.
If you are on the verge of seeing her for just being too much to include in your life, the crap in this letter might help you to be firm in your resolve.
If you still have a place for her, I would call her and say it won't get read, because you will not be told that you cannot discuss things. And that she is NOT to behave like that. And mean it.
My mother sent me a short, unsigned letter last year accusing me of humiliating her. I found it satisfying to write back "you may have felt humiliated, but that was your choice, not my doing, your emotions are your own responsibility". I haven't heard from her since. I have siblings, so actually being able to share this letter was a good thing for me - not in a shitstirring way, but just to remind them that I was the reasonable one.
If you know you cannot deal with what is in the letter, get a counsellor. Because whether you read it or not, you have to move on from her shit.