I just don't feel SM did anything purposefully or premeditated. Call me crazy, and yes she seems very defensive and something happened, but I don't see it premeditated. I do see a happy kid in photos and videos. I suspect an accident happened and she had to cover it up. Perhaps when she was out with him. No idea what it would be that she wouldn't try to get help for him but I think something happened and she panicked and dumped his body somewhere, then returned home, and then concocted something elaborate.
I'd be interested to know where that red truck is now, if there's forensics being done on it.