"so leaving them with this mother when he knew or ought to have known that she was very far from coping well for YEARS is an ok response?"
He hadn't left them, SS and the courts have, everything was reported, they knew what was happening in that house.
Once your children are on a CP plan, you cannot just take them, he couldn't anyway, he had no accommodation and probably (like a lot of these Dad's) couldn't of cared for them.
It was either removal or try to work with the family, sadly the children have been killed (like about four others that week).
If they had of been removed and the Mother sectioned, the DM would be condemning SS, Even more if she had of needed to have a Caesarian applied for.