Brian
You asked for an opinion on the spatial disorientation puzzle as I have put a view on this before I apologise for sounding like a stuck record but here goes:
From your post:
Sir John Day
saying
"Surely the crew could have started to climb, lost control of the aircraft in cloud as a result of spatial disorientation or just bad instrument flying, and then crashed from that?" But the answer was "No"
And:
Interestingly, the BoI concluded, "...therefore, that Spatial Disorientation may have been a contributory factor in the accident."
I put it to you that there were two degrees of spatial disorientation possible in this incident:
The first, as described by Sir John Day, would be the all round loss of references/ horizon, etc as a result in being fully immersed in cloud;
The second is, I believe, the loss of references in one direction (ahead/ right) which resulted in difficulty in judgment of distance off the Mull whose ground detail was obscured by ground hugging mist which merged into the cloud base the horizon, sea surface, etc being clear to the left enabling level flight visually.
The former could be dismissed simply by considering the cloud base which, I believe, was above the crash level never mind their altitude in the final minutes of the approach.
The latter is a possible explanation for the crash or a least a possible contributing factor in two scenarios:
1 If they had technical problems, they could have mistakenly thought that they still had some room to attend to the problem (as opposed to the immediacy of flying the a/c away from imminent danger as discussed earlier on this thread ie allowing distraction);
2 (Here I go again!) Had they been intending to get in close using some special reference that was wrong, their visual judgment may not have been good enough to alarm them to their actual proximity in time.