I think I have another image with a better view of the postcoxal line (I didn't want to overload BugGuide in one fell swoop) and will try to upload it when I can. Nephus sorididus and N. guttulatus are the two members of the genus Nephus that are listed in a 2010 US Navy survey of San Nicolas Island (although there could be others, insofar as our current survey has already turned up quite a few unique taxa in numerous orders that the 2010 report did not).

You may yet find binaevatus (non-native, present on at least some Channel Islands), atramentarius, and quadrarius.

The islands have an endemic Scymnus (falli), so you never know what else might be around. Given high diversity in other families, there must be at least one more endemic Coccinellid. I'd love to check these islands out myself some day.

... the postcoxal line conforms to your description of Nephus, not of Diomus. I'm sorry I don't have another good photograph after all to post here, but I can see the postcoxal line clearly enough under the microscope.