You think counting a kill as shared because two people shot the same person as "pushing it", but counting dozens of unconfirmed, potential off-screen kills and attributing kills to actors when there are no indication presented in the film are totally okay.
And the zombie analogy doesn't fit at all, zombies require to be shot in the head so a body shot would have no effect, on a human it would.
The same situation was also included in The Wild Bunch count as shared between Holden and Borgnine.
I only count off-screen deaths if they're obvious or shown beforehand (not counting the two brief goof-ups in the past)
and just because two people shoot at or harm the person doesn't always mean they were both responsible for that victim's death (Statham holding down Gary Daniels down while Li breaks his neck for instance is not shared). As for the Zombie example, yes, it's a different feature but typically in those movies, several people will shoot one and it wouldn't be counted as shared unless both of them shoot it in the noggin.
Once again, the kill goes to whoever finishes off the victim unless it's hard to tell (both protagonists fire at the same time, etc.) is what was typically agreed upon in the past.