The proximity to Machrihanish airfield is a pure red herring to any hypothesis. Looking at the map, logic dictates that the likely track from the intended Mull turning point (which would probably have been short of the coast) would have been northerly or slightly west of north. This would have had the aircraft flying more than 5nm west of the airfield, which would have put it outside the MATZ, be it civilian (2nm radius) or military (5nm radius) - I cannot remember if the full military MATZ had been de-activated at that time. The crew would have no need to call the tower, since below 500ft, low level, there would have been no conflict with aircraft in the circuit or on the aproach to the easterly runway (even if it was in use at the time). Low level, by definition, is to avoid active airfields; there is no requirement to call local airfields (except where airmanship dictates, eg a very busy site - which Machrihanish wasn't).
Even if the intention was to call the tower, there would have been ample time to dial up the frequncy and call, once the terrain masking had cleared AFTER the turning point.
You are wasting your time arguing the case for or against whether or not the crew could SEE the coast, or flew into IMC - we will never know. The conditions reported by ground eye witnesses as to visibility are also meaningless - the visibility to the crew could easily have been vastly different (observers looking in/out of sun, local effect of hill fog, and so on).
None of your arguments detract from the contention that because we don't have crew statements or other definitive evidence, you cannot prove negligence beyond reasonable doubt.