With a final divorce pending, personally, I wouldn't put ANYTHING in writing to her or anyone else for that matter, as it could be used against you somehow. Stranger things have been known to happen.
Your MIL said her peace. Who knows what info. she's been fed! Plus, she probably put her own spin on things for her own purposes (meaning in defense of her son, not wanting to acknowledge the depth of her son's problems, maybe her feeling his problems are a reflection on her, etc.).
Vent away all you want here, where it is safe, hopefully! For now at least, hold off in responding to her in any way. Just my 2 cents.