Yeah, it's certainly true that if the ping isn't as advertised, then we're all chasing shadows. Although even if it wasn't, one would assume it at least came from Bill's phone.
There's another wrinkle to consider, though. Bill's phone pinged just after sunrise on Sunday but
not before. So that means that Bill's phone was working and still had at least enough battery to boot up from the time he left the trailhead Thursday to that point. The other problem with Smith Water is it means he's in a fairly enclosed space (perhaps 2 miles x 2 miles) for two plus days where there's no cell reception, then once he breaks into a cell area (and there just aren't many on the 10.6 line), he vanishes in an area that's been pretty heavily searched.
Be that as it may, one presumes that Bill is trying to get a phone call out as soon as he gets in trouble, presumably by 7 or 8 p.m. Thursday night. Here's a map from Tom's website, which in turn is borrowed from Verizon's site and I think Google maps, on which I overlaid the four possible places he most likely was at that time (the brown areas are Verizon's stated coverage areas):

Bill's car is parked on the extreme bottom right. Quail Mountain is roughly in the middle. "1" is the cell dead zone between Quail Mountain and SWC, "2' is SWC itself, "3" is the area northwest of Stubbe Spring, and "4" is Upper Covington (where there is no cell coverage, contrary to what this map says, other than at certain spots on the extreme western escarpments). The blue lines are existing trails. In my experience they are fairly accurate around the SWC area but overstated around UC. The SWC "magic bullet" spot I have talked about is at the thinnest band of brown lying between the white areas marked by "1" and "2".
So you see the problem; how is Bill out for 2 1/2 days without hitting a trail and/or a cell tower? This is another reason I was looking at Upper Covington so hard; it was easier to reconcile with those facts. It looked pretty good, but getting him into UC was pretty hard to do, because you can see the road anywhere you enter from the south or east, with one exception - and on that exception, he would have had to jog into more difficult terrain and somehow miss a trail that was just a few feet away.
Thoughts?