I saw my therapist this morning and got her take on M's letter. I don't entirely agree with it, but I think she does bring up something interesting.
With regard to the line of "one day you will learn the truth that I was never the evil mother that you made me out to be," T thinks that even though I never badmouthed my mother, that other relatives observed her mistreatment of me and said things to her, and that she immediately assumed that I had turned them against her. There may be some truth in this, as others DID see it, particularly my Grandmother, who was very vocal, and most likely had it out with M.
Way back when I was about 20, my Grandmother hosted a family reunion back home in Canada. M was going through one of her "silent treatment" periods at the time, and refused to let me attend with the family. My Aunt (Ms sister) paid for my air fare, and I traveled with them. All week long, people were terribly confused over which family I belonged to at this reunion. Cousins who had been born after I left Canada, and didn't know me, assumed that I was my Aunt's daughter, as my own M refused to speak to me the entire time. A few of M's siblings took me aside, and told me for the first time that M had "problems," and that they felt badly for me. I mean, she refused to speak to her own child at a family reunion. That doesn't look particularly good.
So it's possible that some of these relatives may have called her on her behavior, and I was blamed for telling them that she was "evil." I think there are probably other factors at play here, like the possibility of projecting her feelings of me being "evil" back onto herself, but T may be on to something. I was always the bad girl. So if a relative called M on her abuse, she would definitely have concluded that I was badmouthing her.