A gunshot wound would make a huge mess (sorry, that's awful to think about!). But if we can revisit the vehicle swap theory for a minute, perhaps it happened inside a vehicle & the umm, evidence, was contained. I don't remember if we discounted the vehicle swap idea? Anyway, there is that.
If it were accidental I can't imagine them covering it up unless there was something else going on... can't really discuss the options there, but if the parents were distracted by something and the baby got into the car?
The only way that I can consider an accident & then this insane cover up they have tried to concoct is if some other illegal activity were also happening and something made JM/DK paranoid enough to think that the only way to get out of the situation was to cover it up.
Idaho is a gun state & little kids accidentally shoot themselves or each other. It happens when people are careless. The same way little kids get into the medicine cabinet or accidentally run each other over pretending to drive. Parents are irresponsible and inattentive, kids have no idea how dangerous things are and there you go- horrible accident. BUT most parents are "hauling" their kids to the nearest ED screaming for help, even if it is obvious the child is not alive. For a parents first instinct to be "how am I going to make this go away?" is seriously screwed up.