Andrew,
I think you are suffering from a case of taxonomia!  The oldest name is the
right one??  That works in taxonomy, but not in common english; otherwise we
would all be speaking Olde English :-)   I think "mollusk" is still in
commonest usage among the general population, while "mollusc" is usually
used by those of a more scientific bent, presumably because it is derived
from the phylum name Mollusca, which does NOT have any alternate spellings.
Paul M.