Actually, assuming for the sake of argument that Kirk did marry an Agnes Cain, I'll be surprised if there's any evidence that she was ever called Elizabeth.
The trick was, you've got no info on Agnes's parents, you go looking for Cains, you find one, dates don't quite work but nobody will notice, he hasn't got a daughter called Agnes, but there is one called Elizabeth who doesn't seem to have married, so we just say that Agnes was Agnes Elizabeth really and that's her. Brick wall demolished. (Creative Genealogy 101)
There are thousands of those out there.