Hugs to you (((Shelley))) so sorry for the loss of your baby. Yes, you don't need this, it is too much. I am also sorry for his mother and her loss and the guilt placed on your husband, but you know what, they don't need to put that sadness and burden on you. We are supposed to learn from our pain and work on moving forward, if they cannot, it is their lesson to get through. And it is not on you to teach them. Sad as it is, we just can't help some people. They are stuck in their own dark world, full of sorrow and despair, and they won't or can't see all the beauty out there because they can't move past the pain. Very very sad but I think if I were you I'd go ahead and tell him you love him (if you still do) but if he wants a divorce OK, you won't fight him and seriously just wish him well. If he feels he can't be a good husband and a good son at the same time, then best to let him go. Too bad his mother allowed her grief to destroy the life of her living child. Then remind yourself what a huge heart you have and even though that means getting to love deeply unfortunately it also means feeling deep pain. But you can keep going and keep loving and one day someone with a heart as warm as yours will love you back the way you deserve to be loved.
I am sorry you lost your baby. For his mum to even say such a thing is terrible. Your MIL sounds a lot like mine. My MIL lost a daughter before my ex was born and when she had him she focused all on him, to the point of family division, anyhow she wanted to be #! in our marriage, I fought her tooth and nail for 11 years. At our wedding she didn't not smile, not once. She made my life HELL. When we had kids I was afraid she would try to "take over" When they were born she tried to at first but I put a stop to it right off. She was mad because of religious differences and would tell me my children were going to hell. But was sweet as apple pie when my ex was there. She used guilt to make sure that birthday parties family get to gatherings all revolved around her. She actually scheduled a birthday party for her husband on Mother's Day one year so that I had to chose where to be. I was not at the birthday party. She HATED me from day one. I am sure she threw a party when I left her son. LOL
Get out while you can. It will not change. I am sorry for your loss but be glad that you will not have to fight this woman over and over again for the right to be a mother to your own child.