Well. We know there's nowhere to pull a car off the road before you get to the picnic place, so forget that. We know that the ground is very compacted and stony pretty much everywhere in the open around the picnic place. Very difficult to dig and impossible to restore the appearance afterwards. There's very little cover around the birch trees and it's probably no better for digging than in the open - probably worse, with tree roots. The pond was always a possible, but it's not very deep and he probably didn't come equipped for a water disposal. There's no reason for him to have taken her over to the other side of the Allt Broighleachan, there are ample possibilities without doing that.
At the beginning of this I thought he'd buried her, and I thought a commercial forestry plantation was an unlikely disposal site - due to the roots, and the open aspect of most of these under the canopy, and the fact that scheduled felling would be liable to expose a body. I've changed my mind on both of these.
The picnic place is less than six miles from the A85, but it could still take the best part of 20 minutes to get up there. I timed myself at 10 minutes for the 4.35 miles to the bridge, and it could easily take another ten minutes to get up that potholed track (1.4 miles so that would be 9 mph). So that's 40 minutes to subtract from the missing 1 hour 50 minutes. So he has an hour and a half up there, roughly. That is not long enough for a burial, realistically. It could be long enough to get her out of sight, in a hollow perhaps, and cover her over.
He has to change into whatever protective clothing he has brought. Do a recce for a suitable disposal place - I think he'd have done that, and I think that obvious gap in the banking around the plantation, where the burn flows out, would have grabbed his attention.
The ground is very uneven, but he was strong and fit. If he's not going for the pond, it's the nearest place to get her out of sight. It's not a very inviting place for picnickers to take a stroll either. In among the trees he might have been pleasantly surprised to find that it isn't open and there are a lot of possibilities for simply dumping a body. I think he'd go for it.
I think he might have driven the car as close as possible to that gap before unloading her, and he might then have driven it back and parked it neatly by the picnic table before proceeding, to look as unsuspicious as possible. Then he has to do the hard work - get her in among the trees and find a hollow where she won't be noticeable, then try to cover her over with moss, branches, whatever.
We have to remember there is other stuff that must be hidden as well as the body. Whatever he had wrapped her in to keep DNA off the car boot. Duvet, ground-sheet, tent, something like that. Any digging tools he had brought, which would have been useful for covering her over even if he didn't dig a grave. His own protective clothing. We can't know how much of that he took to Lochgilphead and then dumped on the way back, but I'm envisaging the boot lined with something impervious and him working within that space, maybe loading everything back in there, intending to clean or dispose of it all later.
He notices time is getting on, and he has to get changed again and make himself presentable before he gets to the school. He doesn't really have all that long once you factor in getting changed twice and the recce and the time to move her into the trees. I think he could have managed the concealment in that time though.
Then at the school he discovers that the cops are on to him, and he will have no opportunity to get rid of the other stuff after getting home, as planned. What to do? He could take a chance and dump the stuff by the roadside somewhere, but if it's found and connected to his journey, he's toast. Maybe find a council waste disposal facility on the way. Does he know these places often have CCTV to catch unauthorised dumping? Was that even the case in 2010? Maybe he thought he would do that when he said he'd be at Corstorphine by 9.30, but then reconsidered.
The second missing time suggests to me that he decided to go back to the same place, as it all went well the first time and there was scope under the trees to hide more stuff. He has under an hour the second time. Same routine, except he only has the equipment to move into the trees this time. Probably took a bit more time to perfect the concealment.
I hear what you say about area 1, but I have a hunch about it. These fallen trees might not have been fallen 16 years ago - there are other places where we know trees came down in a big storm a few years later. Area 1 just seems inviting, as it turns further away from the picnic place and the continuation of the track to the pine forest. Maybe she's even under the fallen trees, which would be a bugger. But of course area 2 is right in the frame too.
The very least the cops could do, now that felling is underway, is ask the forestry people to take a very good look at what they might be turning up when they get to that place.