I tend to agree with you on everything there except the last line.
Surely it would be really risky to take Jayme on a whim? Yet the perp seems to have gone in shooting.
The house is set back from the road, so I don't think it's entirely ideal for a totally random incident, unless someone followed them home and saw the car turn into the driveway, drove on and returned later with the knowledge that when they came back in the dark they'd have to watch out for that turn off?
I think it's possible that the perp could have gone there with the intention to abuse J while 'he' was there in the house. I also think it's possible that Denise was attacked during the time between the shots and the 911 call. But it seems more likely that the perp went there with the intention of killing the parents and taking J with them? Perhaps had a place ready that they intended to hold J, whether that might be something they'd prepared for or just somewhere that they had access to like a relative's hunting cabin? Especially if the perp was aware of the 911 call, driving off with a live person seems so risky. More so than turning up at a relatively remote location prepared to shoot. After all no one did call 911 because they'd heard shots, no one has reported through msm that they heard the door kicked in.
I can't see the point in a perp driving off with a dead body, especially not a meth-head or other druggie. If they wanted to cover up their presence in the house there are other ways to go about that, and then when they drive off they're 'clean'...at least if they get pulled over they don't have a live or dead body in the trunk.