I certainly agree with David Gowrie's complaints in
Issue 119 about who's ever. And I'm reminded of another very prevalent abuse these
days: using whatever completely alone. As you point out [regarding
whoever], it's a pronoun. A pronoun does not constitute a complete
sentence. We were always taught in school that to qualify as a sentence, an utterance must express a complete thought.
Whatever means "anything or everything that" or "any ... that" [Webster], which
ostensibly lacks a target to complete the thought. It's probably a shortening of "whatever you say" or "whatever the case may be", and even
those aren't really complete thoughts.
Another misuse, particularly of whenever, is now endemic among
schoolchildren. They use it for when: "Whenever I get home today, I'll
call you." Admittedly, whenever could fit properly, with a shift in the
meaning of the sentence (no matter what time I get home). But this isn't what the kids mean in this usage.
Issue 120, page
3