Walter,
Go back and read BOAC 4740.
If the plan is to kill everybody on board the aircraft it is a p**s poor plan.
The aircraft is approaching the coast with a visibility of one to two nms and the lighthouse area is clearly visible (Holbrook). I agree with you that the conditions are conducive to Visual Illusion, but people only succumb to illusion some of the time. Surely these master assassins, who have never come to light, could have devised a more foolproof plan.
Now, if you said that someone on the Mull, who was working with the aircraft, switched on the beacon in the wrong place and the subsequent Visual Illusion contributed to the accident. That I could believe.
But where is the evidence?