This is so great, thanks.
I'm very troubled that a 17 year wasn't missed for such a long time, quite a bit longer than JJ. I think she must have gone first, since she would keep track of JJ, whereas he wouldn't keep track of her. Also, I imagine there was a date that she was intending to enroll in school? It had to be before then, so she wouldn't be kept track of there.
I wonder if police obtained her phone, and have perhaps detected when the usage dramatically changed. Possibly the trip to Yellowstone was a sort of last treat, like a last meal. The cold-heartedness would have been immense. A sleeping drug, followed by suffocation, would be the simplest.
I agree that there was likely a freezer involved, that has long since been disposed of. Perhaps it was in Alex's unused apartment.
IMO, the bodies were buried in Chad's yard because it is agricultural land and easier to dig, and then he could cover it up. Wild land, like National Forest, is extremely hard to dig, there are so many tree roots, rocks, etc. And there's always the risk someone curious will show up, or animals will dig it up again.
I wonder if the night-time bonfires were to encourage a large number of people to walk around over the graves, concealing them.