I read it as “the unexpired term” of the candidate who is no longer in office or, as in this case, dead. So, how does it matter when an appointment is made, if the candidate died more than 2 years and 6 months before the end of the unexpired term?
This will be the interesting part of the process to find a replacement.
It could be argued that the remaining term will be the length of time from when the appointee is actually sworn in as opposed to when the actual vacancy occurred which would be at the death of Sen. of Byrd.
I don’t know if the rule will require calling a special election? The requirement mentions a ‘primary next’ and W VA has already had their primary for the upcoming Nov. election. Lots of unknowns as to the import of how this rule will be applied.
I'd say that the term belongs to the seat, not the person in it. That seat is a part of a class, and the dates of terms are fixed.
-PJ