I think Molly's mother was very attached to the children, but that (like Molly) it was a kind of possessive, entitled attachment. Interesting point above from Logic-Lady, that TM was dressed but Molly was in pyjamas, and I think you are spot on about why Jason's passport was out. I don't think there is any way that TM would have gone along with this kind of plan to kill JC - if he had, it would have been colder and more choreographed. Molly's frenzied rage was their undoing, so she had to have been alone with a sleepy groggy Jason to have been so out of control and not stopped.
TM had absolute contempt for Jason, just as he had contempt for the previous fiance. Look at how he expected that young man on a low wage and poor health to pay all the rent on a property TM owned, where Molly was living. Molly didn't have to pay, someone had to support her! And while TM took that man's money he had contempt for him.
But I don't think his contempt actually gave him a motive to kill Jason. He didn't seem to pay much attention to Molly as an individual. He wanted someone to look after Molly, but from TM's point of view, that was Molly's problem, just as it had been before - she would need to find someone else to look after her if Jason left her. Killing Jason didn't achieve anything from TM's perspective, as I don't think he was so connected to the children. Once Jason was killed though, TM was very invested in protecting Molly for his own sake - he didn't want to tarnish the family reputation, he didn't want to answer any questions about how he and his wife did nothing while Jason was killed. And he had such contempt for Jason, he easily felt that Jason deserved to die, simply for being an inconvenience.