Wow, Mary! I think you should be sainted. You are handling everything so well, and really have a great attitude, really wanting what is best for Erik.
I have a couple of comments. First off, about your MIL calling Eli delayed, if you don't know, Albert Einstein was a delayed talker. He had such a hard time learning to talk that they thought he was "slow" and look at what a genius he turned out to be!!! Here is a link for ya...
http://home.cwru.edu/~sjr16/20th_people_einstein.html
I was in the opposite position from you with the MIL problem. My mother "tolerated" my husband when we first got married. Granted, I had known him only 2 months when I married him, and they had met him only once, but they should have respected my decision. Nope... she put him down every chance she got. It wasn't until one Christmas when we were at their house and she was being especially b*tchy to him when I'd had enough. I said something to the effect that if she didn't want him here, that was fine with me. She misunderstood and thought I meant that HE could leave. She said, "If he wants to leave that's fine by me." I said, "If he goes, I go, and if he's not welcome here, you won't see me or the kids here." I told him to pack up the van, we were going home. He never questioned me, just started putting stuff into the van. Daddy talked with me (through all my tears, because I am not a confrontational person) and asked if we would please stay. Mother was saying something and Dad swatted her on the bottom and told her to get upstairs.
To make a long story short, Dad talked me into staying. Mom told David she would try harder to be nice (after Dad talked privately with her) and things went in an opposite direction from that moment on. My mother thinks the world of David now, and even when we separated last year (after almost 28 years of marriage) she never said one bad word about him and was probably as happy as I was when we reunited.
Dad died in 1992, and to this day I don't know what he said to Mother, but whatever it was made her do a complete change.
All this to say that you are absolutely doing the right thing. It isn't your place to say anything to her. It's Erik's. My heart hurts for you and what you have endured these years. I am sure the therapy will help both of you deal with this.
Keep being the sweetheart you are!