i just wanted to say i am in a very similar position to you, (thread on here about it)
i also cut contact, but for 12 years now.
recently i attempted some contact with my brother, (ex heroin addict with big issues...) and that in turn led to a similar thing happening from my mother - she contacted me through facebook.
i fell to pieces, absolutely.
but i did not respond, i started counselling, and after 6 months of tentative contact with my brother, it dawned on me that they came as a pair because he could not respect my wishes.
i took him off facebook, and he sent an abusive text which has ended any relationship.
so be it.
in your position, i would not, i dont think, justify your self to anyone by sending that letter.
I worked out that i felt powerless and worthless when i lost control of the situation. I feel so much better again now it has at least reached some kind of conclusion, and normal service has been resumed (ie - no contact at all)
I have half expected my brother to do the same and get in touch with my employers, im still giving to some though to that one...not sure whether it will make me look like some kind of mentalist if i mention it to my boss or not....
i got some fantastic advice on my thread, from people who could see it from the "outside" and maintain some perspective, because i felt like i lost perspective entirely for a while back there....it throws your entire safe world back into turmoil.
(i too have kids, a stable marriage, job, all normal, and i have to protect that,)
If you want to warn your mother off pulling a stunt like that again i would have a solicitor send a letter.
if she continues to harass you then there are other things you can do.
My SF is dead. I do not want any contact at all with what remains of that family. I completely know where you are coming from.