I also note that, while financially comfortable myself, I personally could not afford to buy or maintain an ATV. Yet DM owns one, which I am sure, even if purchased used, cost in the thousands. How does he afford it? Which makes me wonder/speculate if there is an alternate stream of income from criminal enterprise. I concede it is possible MBM has some sort of income stream.
If there is an association with a criminal element, even if MBM stood by while DM was involved in it, and it exposed her children to a dangerous element, it could be difficult for MBM to confess this to law enforcement, given that it could jeopardize her custody of her youngest daughter.
This might explain why MBM would go along with DM in terms of telling the same story about what happened that morning. It would also explain her hasty end to the relationship with DM. She may have found herself to be between a rock and a hard place.
This is just me casting about for possible scenarios.
IMO