You sound like a good woman. From reading your comments, I do think there's no going back for the marriage. He's a completely different person to you now, and the loss of respect is massive.
If I were you, I'd probably focus (other than on your new baby of course), on building and maintaining a positive relationship with the OW, you never know, you may both become good friends, and letting all of the siblings grow up openly having a sibling relationship with one another.
There's good that can come out of this with that.
As for the MIL, I'd likely let the OW know that MIL wants a relationship with the baby, see if she would be open to that, and if she is, then I'd tell the MIL that OW would welcome her having a relationship with her Grandson, that will hopefully make her think twice about following her son's instruction. Honestly, he's just awful, I'm so glad you're not taking him back.
You're doing really well, you're a great role model for your children, as well as your children's half-sibling.