Vec and SFFP
I think you are both in violent agreement with eachother!
Of course, any sane person can see that, without CVR/ADR, there will always remain an element of doubt as to what actually happened.
Regards (IMC) safety altitude, it would have been be good airmanship to be at the SAlt for the next leg BEFORE the next turning point (remember 'pre HAT' checks anyone?). This is what I was taught when a student and later on the OCU and what I, in turn, said as an instructor. If the SAlt for the next leg was 5900ft, it meant that, if trying to climb and 'fly the plan' at IMC altitudes, ZD576 would have to have climbed to 5900ft amsl on QNH before they got to the Mull! 5900ft would then have sensibly become FL65 (extra 450 ft due low local QNH of 998mb) and thence FL75 for the correct quadrantal for the leg to Mull/FL65 again for the leg to Corran. If the crew were using the Belfast RPS (highly likely) and it was 996mb or below, then it would have been good airmanship for the 5900ft to have become FL75 to Mull and FL85 to Corran .....all of which are easily above icing layer of approx 2500ft (don't forget the OAT guage was only accurate to +/- 1 deg C) - which is why the IMC option never existed. I suspect that the crew were well aware of this and why they were mightily cheesed off that the Mk1 was u/s - not only for the icing limitation but also as the Mk2 nav kit was not cleared for use in IMC in the fabrication of an RTS!
This then begs the question - why try a cruise clmib at all? (as the explanation that the BoI came up with). I don't believe the crew would ever have considered this option, as I understand the SH procedure was to slow down/turn back, do a turn emergency 180 deg or, in extremis, do an rapid max climb to the emergency safe altitude - in Scotland, generally held to be 6500ft amsl (Ben Nevis). Obviously, ZD576 was doing none of things - so either they were happy with the viz and their visual position relative to the Mull, there was a visual illusion, distraction or disorientation....or they couldn't manoeuvre for some reason (UFCM or Engine/FADEC problem).
Doubt? Oh yes, mountains of it!