Again I think this all may tie into Molly's state of mind at the time of the murder. Molly obviously came from an educated home, both of her parents were professionals, her brothers each successful in their own fields. Molly herself had started at uni but then dropped out, potentially she left uni and started working at a hair dressers/hair academy and seeing this guy Jeremiah. This relationship and career path fails. She meets Keith McGinn...again they get engaged quickly and move in together. We know her parents had reservations about this relationship, and from what I understand from the excerpt from the book, this relationship ends spectacularly badly.
The life that she built with Jason and the kids was finally giving her the opportunity to prove herself, that she could be successful at something. That she could stick with something and make it work. Her whole life was built upon this perfect family she had created - she was a swim coach, a teacher's aide, she helped out on the local committees with the other Moms. If Jason divorced her, she would have lost not just him and the kids but she would lose everything. Her entire world had been built upon the lifestyle of being an upper middle-class stay at home Mom, she would have lost her entire identity if this marriage failed. If it was her understanding that she could not cope with this loss that prevented Molly from divorcing Jason the year previously, how would that desperation have presented itself if Jason had finally told her the decision had been taken out of her hands?
I also don't think we can underestimate the role her parents played in this scenario. I think Molly strove to show her parents she had finally come good. She had finally built a life they could be proud of her for. (I was surprised when I saw a picture of the family home in Knoxville, the Panther Creek house is almost a carbon copy of the front elevation) I believe Molly was under immense pressure from her parents to finally get control of her life, to finally stop disappointing them. I wonder how she could have coped if things started to slip. If her mental health issues were recurring, if her marriage was in trouble, could she have confided in them? Would she have had enough of a support system to help her through this or was she really looking at a future that was just a black hole, where everything she had worked for had just evaporated?