Like all of us, I've been thinking....
When was the business with the cellphones planned?
For me, this kind of falls into two parts. The first part is when PF picks up Baby K and KB is never seen alive again. He disposed of her body and he could have stopped there - go home, play dumb, and get on with life - assuming he didn't get caught and he didn't plan to.
The second part is the phone business, designed to buy time and lead investigators on a wild goose chase somewhere between Gooding, Idaho and KB's grandmother's or parents' homes. That was 100% KK's part, because PF wanted to stay close to home. That would be his alibi and everyone would assume KB had gone missing somewhere in Idaho.
Did PF come up with this phone plan after he killed KB? Because if he did, I think he was still trying to set KK up. I say 'still' because he was trying to keep his hands clean and get her to do the dirty work in the first place and I don't think it was because he was squeamish.
He could tell the authorities he thought KB was going to see a sick grandma, but if the pressure got intense, he could mention that he did happen to have a jealous recently divorced ex-gf. Of course, when the cops asked where she lived, big surprise, she lives half an hour away from where the phone pinged and KB presumably disappeared.
The cellphone business always bothered me, particularly because the story would have worked much better (KB driving to Grandma's) if one of her cars was gone. He had to know it wouldn't buy much time - a week or two at most - and then the authorities would be called in. Was he trying to buy time because somehow it lessened the chances that KB's body would be found?
That would be true of the landfill, if he put her there. The longer it took them to start a search, the harder it would be to find anything. However, for some reason, I just don't think he took her to the landfill. Too many other places he must know of around there.