I think Bryce was with the car. If it were some assailant, I could see leaving the phone, but they'd take the laptop for sure, and wallet if it had any money or credit cards (see my response on money below). If Bryce assumed he was coming back to the car (or assumed he wasn't), he might leave his things in the car.
Which means Bryce was most likely driving at the time he was speaking to his mother. This could be the reason he turned around the first time, he was sidetracked by speaking to his mother, and didn't originally intend on going that far or that way.
I am having trouble following the reports. Originally it was reported that he ran out of gas at the Buttonwillow rest stop. He ran out of gas at 9:00am, but the guy with the gas didn't arrive until 12:00pm (3 hours-seems like a long time). If he got back on the road at 12:30, , he'd have arrived at the TA Travel center (on Lagoon drive) around 12:45. Or he might have tried to sleep some more at the rest stop before heading to Buttonwillow proper. If he left Lagoon Dr. around 11:00 PM ish, he was there for around 11 hours or less. Fill up gas tank, use facilities, maybe grab something to eat would take around an hour to 2 hours. 8.5-9.5 hours unaccounted for. He may have tried to sleep, and didn't get much due to comings, goings, heat, light, etc.
Sgt Wallace was replaced with some other LE. That tells me a lot.... But again, an emergency tool leaves a distinctive pattern when used. I wonder if what Yellow Camaro saw could be correct, but with going forward? Stuck with the rear wheels spinning, but trying to back up the hill, while the nose was pointing downhill?
Or possibly he was using his card and saving his cash for when he ended up places that only take cash. I do that on trips all the time. You end up at JoeBobs gas station in the middle of nowhere because your fan belt gave out and by George, JoeBob only takes cash. Bryce may not have been that savvy, but maybe he was.
If his mom kept calling every hour, legally he'd have had to pull over to talk to her (unless he had a hands free thing going in his car). It could take that long if he did. There is also a rest area at Frazier Park (Lebec?) at the top of Grapevine hill, he might have stopped there for a bit. That's a bit odd; if it had been by itself, not so much, but with everything else...maybe.
BBM: If I were him, I'd have been sleeping in the back of the car. I'm pretty sure you can put down the back rows of seats and have a pretty flat spot to sleep in. The back windows are usually tinted, so the road service guy might not have seen him. Most people waiting for gas are standing outside the car, chomping at the bit to get going. I can't imagine anyone sleeping anywhere outside in the Buttonwillow rest stop, but there are covered picnic benches for shade.