Op please do not send photos of your child op.
If your ex and his mother reconnect, and they may well do, its his mother after all - she may well show him everything you have sent her, and restart his desire to see his child and be a part of his life, causing no end of problems for you potentially down the line. Especially if in court he can prove he has worked hard to change himself, and has a right to access - even supervised contact.
Do not send anything to do with your child. She has other grandchildren she can focus on. You do not need to do anything.
In your position, I would send a thank you card once or twice a year if she sends gifts and outline you are getting professional help and support and can not have any contact with her or anyone for now, don't get into the detail of your life or your son's life or progress. Simply thank her for her kindness and leave it there. If want to. And even this might be too much for you. I would then block her, and would not get into a pen pal situation.
As sad as it is, this is her cross to bear op, not yours. She has other things she can focus on. I expect much of this stems from guilt, which is misplaced, because it is not her fault her son has turned into a monster.
You and baby have to come first now, if you are ever to be safe, and that is all that can matters right now. Your protection.
As tempting as it is to invite her into your lives, I can not emphasis how potentially difficult this could prove to undo later on. Draw a line and focus on safe people - family, friends and neighbours - the community so you can have loving connections without the risk.