Walter,
As an experienced helicopter engineer I have not intruded on this site as it is all about piloting so far but the thread is now getting technical on altimeters.
Is it not possible that the QNH and the QFE for your surmised LZ on the Mull were the same? I am not discounting your theory as to my mind anything is possible unless proved otherwise and I have some experience in helicopter accident investigations with Bristow Helicopters.
Unless there was, for instance, a helicopter at your landing site setting his altimeter at the field height and advising the Chinook the QFE how would the Chinook know the altimeter setting for QFE for the LZ? This must depend on the barometric pressure of that area at the time of the flight, I cannot face going back through all the posts to find out that information so forgive me. It is difficult enough reading this thread with all the comments being made on such a complicated accident and there is no index in the threads to quickly find a post.