Oh dear, I 'm so so so sorry for what she put your son through and I don't blame you for being so upset. Who wouldn't be?
I guess the task is to find a way to have peace and harmony for the rest of your life. What will happen when you and your FH have children together? Will your present children not be allowed contact? Will the new ones not know gma?
It's a hard situation. I would guess that she has NO CLUE how to be respectful and nurturing to your child (Seriously, what she did was terrible!) and needs to be taught. The only way a person will be receptive to teaching is if it is done with respect for her ability to understand and do it right. Is it possible that you can invite her to joiin you for a session with a therapist, so that she can have her questions answered and an expert who is not emotionally involved can help? Maybe if she goes to a support group she can see how other parents deal with these issues? She seems like she just is clueless and needs to be taught. Are there parenting classes for families who have children with the same issues as your child?
I think the best tactic is to calmly and compassionately -- and I know that part will be hard, but this is for the best -- talk to her about why you were upset and tell her that this is a learning curve for her and based on your child's needs, you think she handled it incorrectly. Tell her that you have made mistakes, too, and you don't blame her. She was trying her best. -- grit your teeth if you have to and say that-- and you'd like to invite her to be a part of the navigation process of raising your child. Then invite her to a learning experience of some sort.
I'm so sorry she did this. It is sooooooooooo wrong. I hope your family can find harmony and peace for the future.