I have been up to the north side of SWC a few times. The one spot where there's known reception around 10.6 has been pretty well covered; I've been up there and so has Tom a few times.
One thing I have to say about the north slope of SWC: the climb up SUCKS. It's really bad from almost any angle. Very steep and tough. It's just hard to imagine a non-able bodied hiker making it too far up the slopes. The last trip up that way I found a back way in that wasn't too bad, but one of the things I was thinking at that time is that Bill might have found a viable entry point to the north side of SWC at the eastern mouth by Quail Wash. It seemed to both Tom and myself (the latter from a distance) that it might have been easier.
I did finally get to take a look at that approach on this last trip and it wasn't too bad. Could Bill have gone up that way? Yes. But.
I am really sympathetic to the idea that it's the most likely place to look simply because it hasn't been looked at much (and you're absolutely right, if he is up there there's an almost endless number of places he could be), and it's about the right place.
The problem goes back to the whole, you're looking at the topography, trying to work yourself up to look at it and you go "there's just no way." This is why I ruled out the badlands west of Upper Covington after I tried multiple times to get in there. Past a certain point, there are just too many obstacles even for an able-bodied person. (Tom did get in, and cursed me by name for advancing this theory when he did)
So...yeah. The slopes of the north side of SWC are just like that. I did climb up to the top from SWC once. I made it. It was long and fricking steep. I will allow that we found an abandoned Snapple bottle part way up, so someone else did it at some point. But it's just hard to credit for a guy in Bill's position. I can accept he might have made it up a little way, and I think that's how Tom sees it.
The mountain lion theory makes a lot of sense BUT - if Bill is taken out by a mountain lion does he get dragged out of the search area in such a way that no trace is left, including his pack, etc.? That's a little harder to credit. And remember - the same mountain lion has to drag poor Bill up the same steep slope.
None of this is to say that he couldn't be there. I'm just speaking from the vantage point of the guy who's looked at some of these spots and kind of gone "nah." And by saying that, understand I have to decide how to expend my time and energy. There is a motivation factor, and this isn't a grid search. If it just doesn't look or seem plausible to you, it's hard to go charging in there.
These are all great suggestions though, keep 'em coming. I'm liking the east side of Quail Wash as the first place to look though. I may have a window to do it as soon as next week.