Just to throw some local experence into the discussion.
With a northly 15 knts at INS I would expect some pretty horrible mountain wave around that area getting towards +- 1000ft/min if not more.
Sub 5.0A - 2.5A in Deeside would have been pretty horrible for rotar.
Over the Cairngorms it can be quite deceptive with slots between the hill tops and cloud. With a northerly it would be quite possible to have 300ft of slot between the clouds and the tops. If you routed Glen Esk, Driesh over into Deeside it might not look to bad as the air will have lost alot of its moisture on its first lift at Cairngorm / Macdui if you didn't know there was going to be a wall of cloud in the Spey valley it would be very easy to presume the conditions were similar to deeside. It would have been quite good fun up up to that point.
Sad thing is if he had bitten the bullet and climbed on a northerly track he was already past the peaks and proberly would only have had 3-4 mins of IMC while benefiting from the lift from the mountain wave before breaking clear of it.
Its not the first time pilots (myself included) have been caught out by the conditions in the area which intially look quite good but quicky turn to poo.
I wouldn't be suprised if he crashed very close to where the american planes went in.