With respect to the ex-husband... I refuse to believe that people cannot change. He did not have to participate in this show. When people have suffered abuse at the hands of non-present family members, the show has typically beeped their names and been vague about their identities. Even if Abbie had specifically named her ex-husband as her abuser, her credibility is not the greatest at this particular moment. Domestic violence is often a family disease, like alcoholism. It takes guts for him to voluntarily put his face and name on television, where it will surely be seen by all his friends, family, coworkers, current and Bpotential employers, etc. and to admit what he did without trying to minimize it or make excuses. I assume he did that because a) he cares about his daughter and knows it is best for her if her mom gets help and that his presence may be necessary to making that happen, and b) it's part of his process of owning what he did and trying to make amends.
I'm not excusing what he did, but it sounds like Abbie's parents are keeping pretty close watch on him and allowing him the opportunity to prove that his treatment and therapy have made a difference. It's probably better for their granddaughter to grow up with a father who she knows (or will eventually learn) was abusive when she was younger but then got help to overcome it and became a good dad than to grow up with no dad hearing only that he was a violent, abusive monster who isn't allowed around because he's too unfit for human contact. This is especially true given that it's unclear at this point that she's going to have a mother in her life either.
And yes, he could probably get--and may already have--court-ordered visitation as long as he can show the court he's doing what he's supposed to do, and under those circumstances the family is probably better off trying to work with him in a controlled environment rather than setting themselves and their granddaughter up for a lifetime of destructive open warfare. It's easy to say in theory that we have zero tolerance for domestic abusers, but the reality is more complicated, especially when there are kids involved. The ex-husband has done immeasurable damage to his family. So has Abbie, at this point, as have almost all the parent-addicts we've seen on this show over the years. Banishment can't be the answer for all of them.