I read the letter to her and her reply, I'll be honest here.
Take the kids if at all possible, this was all said in the other thread by many of us. I see a very irrational person in her reply to you. I see her twist what is happening in her reply. I see my STBXW in her reply.
You have your head on straight, she doesn't. Do what is right for the kids, saving your M comes second to the kids.