JP:
have you studied Annexes J and K to the MOD Response to the Mull Group submission, showing the intended track and the actual track
I don't have the ref to hand, but question your (and Caz's) claim to have any idea of the intended track.
The waypoint and the lines to it and away from it are not used in SH to indicate the intended track, simply a guide to give the TANS information to assist in steering guidance to the vicinity of the W/P.
The flight was VFR, the weather was VMC (Holbrook's testimony confirms).
Why the chinook continued more or less straight into the Mull, instead of turning left to follow the coast below the cloud will never be known without doubt.
That's the crux. How many times does it need to be spelled out!
And for those new to the thread, who wonder why a climb to MSA was not initiated: Chinook Mk2 icing clearance 4Deg C. MSA 2800ft (from memory) 0 Deg isotherm about 2500ft.