Reading all this with interest... TM's motivation is the most mystifying in all of this. He had shown no sign before of being invested in his daughter's welfare... yet one way or another, he "stepped up" for her that night in the worst possible way. I think the two factors would be his own self-preservation - Molly was doing something that would reflect badly on him unless he took control of the situation; and his seething jealousy of his son in law.
With their history of shuffling Molly and her problems into the background of their lives, it would have been more typical IMO if TM had said to SM that day: "No, I don't care if she's called six times - we're not going there." It was a 4 (?) hour drive - he'd left Molly to handle her problems alone before. If he had left her alone, what would she have done? Maybe killed JC in a mad rage anyway, and with nobody to coach her in a cover story. If TM had not been on the premises, I am convinced that they would have left her to plead diminished responsibility, and her social media campaign would have been less structured and supported. If he didn't need to protect himself, he wouldn't have been so concerned about protecting her.
Someone else commented a few pages back, maybe the reason the Ms came that night was because she was threatening to do something that would implicate them anyway - such as abduct the children. So TM shows up, the big man making a front of protecting/ supporting his daughter, when in reality he's protecting himself. They tell her not to do it and go to bed. She's crazy with desperation and anger, goes into a homicidal rage.
Then Act II of Tom needing to protect himself while pretending to protect Molly; while at the same time, he can finally act out his own jealous rage against this man who stepped into the role of provider and protector in Molly's life.
I don't think for a minute that TM anticipated this or helped arrange it, only because it would have been planned more tidily. But I do think he could have anticipated that M was dangerous that night. Maybe before he went to bed, he said something about how it would be easier if Jason was gone/ dead, then she could just keep the children. I think the idea was there, within all of them, and then he woke up to hear a thump and realized that Molly had acted on it. I don't think he realized how badly she had already injured Jason by the time he got there, but I do think he enthusiastically participated for the sake of making it look like two people "fought back." This was another point where he had the option of acting differently, but I think he wanted to "win" over Jason, get the children for Molly and sully Jason's name.
He was jealous of Jason, he would have resented Jason's success and Jason's position in Molly's life, and his jealousy would have fanned the flames of Molly's jealousy. As a parent, he could have understood Jason's stance about adoption - Molly's parents could have reasoned with her, and told her it didn't matter that she wasn't legally the children's mother. She was (at one point, anyway) a beloved involved stepmother. As a wife, Molly could have told her parents to show respect for her husband or step into the background of their lives. But they encouraged each other's resentment and jealousy.
So TM's jealousy fuelled MM's obsession until between them, they destroyed Jason.