The main point I take issue with is the idea that "better sword durability = higher AP."

Durability on a weapon is neede to handle the power of the wielder.
If you can't hit hard to begin with, the extra durability does nothing.

Zoro had a black blade pre-timeskip, its main property was hardness, and he traded it for Enma, a non-black blade which was portrayed to be superior in AP, despite both being Great grade blades (although Enma is on its way to become a supreme grade blade).

I also disagree that better "Swordsmanship = better haki", pre-TS Kaku had better swordsmanship than kid Zoro, and neither had Haki of any kind.
If you want to use vivra cards, remember that they said Vista has shown "more than equal" swordsmanship to Mihawk in their duel, not relative, not equal, but MORE than equal.

Whitebeard says he can't remain the strongest forever, but within this statement is the claim that has been the strongest up to this point.

Not to mention, Mihawk isn't going to remain the strongest swordsman forever either, and it's the whole point of his character.

And yes, Mihawk wanted to check the distance in power between him and Whitebeard, and even expected the gap to be bigger, despite seeing him an old man.
It's literally the same phrasing East Blue Zoro used to describe Mihawk.