Involving the phone's final ping, two thoughts keep circling my mind--
1. Was it deliberate, to lead everyone to that area? (Host of things we could use to support or refute this theory!)
2. Was it to cover his own tail, as in wanting to power it back in to check something one final time-- maybe to see if someone had sent her any new messages in the time that had passed? (seems more likely?)
One of the things that's puzzling is the fact that EA left the SOL area, drove somewhere, and then returned about 0:45 later. Why would he do that?
Here's an idea.
Maybe in that 0:55 he went somewhere, whatever happened between him and her occurred, and then she was out of his car. Whether handed off, confined, killed, or whatever, but she was disposed of from his car. Then he took her phone and perhaps some personal effects, and made the return back to the SOL area, so that if they were found but she was not, it would appear that she had never gone anywhere (an attempt to bolster his alibi that he left and she was still there when he did).
Say he went east on 121, but then avoided the toll-road going back the other way. In that case it would have taken longer to do the return trip, so maybe it was 10-15 minutes going, 10-15 minutes at wherever, and 15-25 min return.
If this makes sense, then if I was searching, I would search for CM somewhere off to the east, going up 121 and see where I can get in 15 minutes-ish with no traffic in the middle of the night. And I would look for and expect to find some of her stuff, incl her phone, in the pertinent area serviced by the tower where that final ping at 4:47 was heard.