I may be wrong about this, but I'm really not completely convinced that vegetation and/or soil analysis is reliable enough to justify concentrating exclusively on the Rest and Be Thankful / Hell's Glen area if it's the only piece of evidence you have for that.
The reports about a cell-phone ping around Glen Croe seem to be wrong. The report of someone seeing a car that might have been Gilroy's could be anything. Lots of silver cars around. Forgive me, but I have up-close-and-personal experience of the Scottish police becoming fixated on a particular place as being the clue to a particular crime, on pretty tenuous reasoning, when absolutely gobsmackingly clear evidence pointing to a different place was right under their noses all the time. And this could be another one.
The route he seems to have taken and these timings (plus the time he was known to be at Lochgilphead) are really all the concrete evidence there is. And that concrete evidence says somewhere between Tyndrum and Inveraray, in the Glen Lochy or Glen Aray area, to me. I'm astonished there's no evidence of a serious search round that location.
The gaps in the journey, both times, are between Inveraray and Tyndrum. Now, if you're going to hide a body in the Glen Croe, Glen Kinglas or Hell's Glen area, you simply don't go that way. The time you lose doubling back from Inveraray into the Argyll Forest, is unacceptable - possibly as much as an hour's extra driving both times. That's putting a serious crimp in the time available to conceal the body. There's also the possibility of being seen there, so far off the direct road to Lochgilphead he has ostensibly chosen.
When we discussed this earlier in the thread, we speculated that Gilroy was driving his usual route to Lochgilphead (nothing at all strange about going via Tyndrum), racking his brains as to where he could stash the body, and seeing a lot of the possible access roads into the forested area closed off by gates. Then he had a sudden brainwave about the area west of Arrochar where he knew somewhere that would be suitable, and decided to detour there even though it took him off his road and would cost quite a lot of time.
This doesn't stack up for the way back though. If he stashed the body there but still needed to do something to conceal it, or for whatever reason he needed to revisit the place, why do the senseless detour all over again? Why not simply go back over the Rest and Be Thankful and be done with it? Much less lost time to make it all look suspicious, and if that's the route he took - and again it's not an unreasonable route for that journey in itself - why then, it doesn't matter if his car's seen. Whereas if he's trying to pretend he went straight from Inveraray to Tyndrum, being seen in Glen Croe is a bit of a problem, if it happens.
And for goodness sake, look at the rest of the return journey. For some reason we don't really understand, he turned south-west at Crianlarich and went down the west side of Loch Lomond instead of taking the obvious route straight on through Lochearnhead to Stirling. This takes him back to within a very few miles of the Rest and Be Thankful, which is a hell of a weird thing to do if he's trying to draw attention away from there. It's almost a circle, for no apparent reason at all.
I'm speculating about whether the Rest and Be Thankful thing might be as big a red herring here as Malta was in the Lockerbie investigation. Similarly, the uninvolved student finds himself wondering, well, why on earth do they think the crime happened there, and did they even look at this huge pile of suggestive evidence pointing to the crime having happened in Heathrow (Glen Aray)? So forgive me if this is unjustified, but these guys have form.
In any case, the problems of disposing of a body are similar just about anywhere around there. Actually finding somewhere where you can take a car sufficiently off-road to be able to conceal it well enough. I'm a bit doubtful of this idea that he knew the Argyll Forest well from previous family holidays and suddenly thought of the perfect spot. And if that was what happened, why on earth retrace his route all the way back to Inveraray and up to Tyndrum, only to end up on the west bank of Loch Lomond, when he could have driven straight there in about ten minutes?
I don't know why he went Ardlui, Balloch and Drymen rather than Lochearnhead (going from Crianlarich to Stirling), but it probably doesn't add a huge amount of time to the journey. On the other hand, going from the Rest and Be Thankful to Tarbet via Inveraray (which he already passed through on his way from Lochgilphead), Tyndrum and Crianlarich, which is what the police theory has him doing, is completely insane.
On the other hand, if we discount the vegetation/soil analysis as possibly misleading and just look at the route, there are equally promising possibilities along the A85 or the A819, or the minor roads leading off these roads. Since there doesn't seem to have been a systematic search there at all, you'd be relying on the body being stumbled over by chance. And it's a big area. A sheepdog or a walker's dog might turn something up, but on the other hand it might not. Or the owner might call the dog away without going to see what it was investigating.
Just my thoughts, but I'm really not that enamoured of the Rest and Be Thankful theory if all they have is the vegetation and soil analysis.