And let us refresh our understanding of the most probable weather conditions that they would have been facing in the immediate vicinity of the Mull (as I have tried to describe in past posts):
Here is a profile shot (from the north) of the Mull in gentle wind conditions that did not produce the surface mist (due to phenomenon of "speed up" as I have previously described) - the orographic cloud has a well defined base and, if you look at the photo closely, you can see that it extends a significant distance out to sea into the wind - in the immediate vicinity of the Mull that day, they would have been underneath this base - a grey ceiling that met with the landmass and prevented visual judgment of range from large topographic features on the hill line ahead:
On the day, the wind was strong and would have been expected (given the humidity etc) to have given rise to the characteristic ground hugging mist in the cooling late afternoon/evening. The following picture is from up wind showing such mist forming on the slopes of Ailsa Craig - note how it follows the ground.
The slopes of the Mull that day, beneath the orographic cloud proper, would have appeared thus from the level of the lighthouse wall and covered the ground, thickening, until it merged with the oro cloud.
They would have been able to see the Mull landmass from a good distance at low level but close in they would have had difficulty judging their distance off visually - and they may not have had confidence enough in their visual judgment to over rule what any other (eg radio) reference was telling them if it was wrongly placed.
