Agree re peat making possible discoveries very unlikely.
Also agree re coastal plains etc being first places to settle etc.
It's also clear that the slope levels would mitigate against woodland in a lot of the highlands.
That being said, I don't think Rannoch would have been that bad a place in better climes, it's a broad area so would not suffer from the lack of sun in deep valley areas.
Even recently, at least before the clearances, a lot of inland Scotland was more populated than at present where most locals live where their recent ancestors were relocated to make way for sheep.
I've no doubt that the better areas you list were populated first and had the best farmland/grazing/wildlife but I think we get a poor view of the potential for use in pre-history when we look at the current landscape in a lot of the highlands?
Mac