despite the fact that they were IMC below their safety altitude
We should be careful with statements like this. Even this is not an established fact. Sure, they impacted the mull in cloud, but just before, they were visible to the Yachtsman, as was the light house.
You are right, we will never know what happened to the degree required to aportion blame.
(although, despite the pitiful icing clearance there still remains the problem of Captaincy, authorisation to consider).
Could you expand on that? I'm not quite getting your point.