This is an old thread but I have thought about this case for years now. I think, initially at least, that JP took SP out I-80 to an area he had scouted in advance. I don't think it was out into the area (Pony Express Route (PER)) that he claimed he spent the night with the kids. Though I think he went out I-80, buried SP and then went the the PER. The kids stated that they "didn't even get to have a fire and make s'mores" I think because JP was too busy most of the night burying SP and traveling to his "camp-site" so that by the time he got to the PER and Simpson Springs it was very early in the morning. I've also thought that this was the reason he made trips out to Wendover on I-80 shortly after this blew up in his face, because I think he was trying to see if LE was looking in the initial burial site. I'm still not sure if JP moved her at a later date. Seems too risky, but who knows?
After rereading my post and listening to the Cold podcast, I have a little more to add to my previous post.
JP is alone most days with SP at work and the kids at daycare. He has ample time to pick when, where and how he will kill SP and how and where he will dispose of her body. I believe this is what happened:
On Dec. 6th, JP cooks lunch/dinner for SP and puts prescription medication in her food to poison her. She goes to her bedroom and her friend (JO) leaves. JP takes the kids out of the house to go sledding. I think this is to make sure the kids don’t interfere and see SP dying. When JP returns he finds SP has moved to the couch. She is either deceased or very sick. I believe she has vomited and JP cleans this up (explains the wet carpet, the fans and the lack of blood evidence). If she is not quite dead, I believe JP strangles SP to finish the job. At this point, I think JP “medicates” the kids using something like Benadryl to help them sleep. JP then takes SP, the camping gear, any supplies he needs and places them in his van. He loads the kids into the van and leaves the house. I believe he heads west on I-80 to a preplanned spot, well away from the freeway and hidden enough that if he uses light from the car or any other source, he will not be seen. He buries SP and then heads to the Pony Express Route and Simpson Springs to camp for what remains of the night. From there he returns to West Valley City, turns his phone on too early, and is seen by the neighbor; his first major mistakes. He then drives south to I-15 to the Utah/Salt Lake County line, turns on his cell-phone and calls SP’s work to establish his alibi.
Later, when he rents the car, he may have moved SP but I don’t think he did. Far too risky when he knows the police are at least very suspicious about his activity. As I said above, I think he drives west on I-80 several times to see if there is any activity where he has buried SP. It's easy enough after the snowfall, all he has to do is look for tracks in the snow. No need to actually go off the highway and out into the desert.
From there – Who knows? JP may never have taken SP anywhere west of his home. There is just no evidence that she is anywhere specific like Simpson Springs. She could be in eastern Nevada or southern Idaho, really almost anywhere within a few hours from his house. The only thing we know is that he is spotted in the Simpson Springs area on the morning of Dec. 7th.
I think the previous discussion of hiding a body in a mine shaft becomes a red herring that WVC PD is forced to check on even though it was a passing comment from JP years before at a party. In fact, it ends up helping him because SP is not in a mine shaft in the west desert of Utah. Very convenient for JP since that is where he wants his alibi to lead the police. (Or at least to the Simpson Springs area.)
JP is fairly intelligent, he is far from infallible. One thing I did notice from listening to his police interviews and which was mentioned in the Cold podcast is that he talks about the truth and shuts up when he can’t tell the truth. This isn’t strictly true but he dances around the truth when he must, otherwise, he goes silent.
I’m still hoping someone happens across Susan’s remains. Hopefully, before her parents pass on.