My big problem with this is that JH has already had many bites of that apple, his second chance; why should MH be his test subject to see if he WANTS or CAN be a responsible and loving parent? That is way too great a risk for her, imo.
He is a 32 y.o. grown man already! Do you truly think he might change bc he LET his son suffer abuse to the pt of death (and is somehow shocked/guilted into good parenting by that)? I just don't see it.
I love that you have a generous outlook (big heart) and see an optimistic outcome with this seriously flawed person. I do. However, I'd never again allow him the opportunity to neglect another child. MH, especially, should not be positioned as his lab rat or litmus test. This is exactly how kids keep getting hurt: putting them back with a proven neglectful parent, hoping this time will be different. Moooooooooooo.
No disrespect intended!