the whole time I had been thinking they lied about the timeline and JM either caused the accident (or murder) or just covered it up for SM, then went out and met Jess, which I could understand her agreeing to lie about if she believed her parents and thought it was just a disappearance and probably suspected JM would easily face some prejudice and harsher questioning. Early on you probably expect theyd find him soon enough and want all the attention on that too, I would. But I did also wonder, if the timeline actually is correct, if JM left a gun out and SM panicked and hid him, then JM moved the body later.
I personally think the cause of death has to be something there was so clearly no recovery from, or I do think someone would try to help. then they covered it up to avoid legal consequences, the family being torn apart, even just wanting to shield Jess from that I somewhat understand the impulsive logic, or out of fear/control possibly if it's a family with DV history. JMO