Saint Shanks, as far as I am aware, has NEVER been implied to have ever been anything more than a bench warmer for the REAL future King of the Pirates. Be that Ace (initially at least) or later on Luffy? Does not really matter, it sure as hell was not intended to be Saint Shanks.
If Roger thought Saint Shanks was worthy of being the next King? Hell, if Saint Shanks thought he was worthy for that matter? Luffy would have never even be able to eat the damn Nika fruit in the first place. Saint Shanks would have eaten the fruit himself if Roger not only chose him as a succesor but clued him into what he was missing.
Also, I am not petty enough to argue Oda knew all the way back in Chapter 1 what Haki was? From a PURELY in-universe perspective if Saint Shanks at a young age unlocked Haki him getting tricked by a damn smoke bomb and literal backwater mountain bandits fodder almost running away with the Nika fruit (and Luffy) is a COLOSSAL L on his record even more so than it already is in canon.
Like straight up you basically have to ignore Saint Shanks intro as pure plot because it makes NO sense that a guy who at this stage of his life had bounty over a billion on his head did let somebody like Higuma get away with the shit he did.
As for the thread? I still think Mihawk could just be a prodigy that turned his blade black early in life and that would fit his character without to many issues. Hell, Mihawk was older during his rivalry with Saint Shanks than Zoro will be when he makes his swords black. Unless I am messing the ages up Mihawk turning Yoru black at 29 is not impossible or even all that unlikely.
Its not like Mihawk and Saint Shanks had their duels while both were 16 or some shit. Both were already grown men, with Mihawk already having made a name for himself WAY before Saint Shanks. Him already having a black blade prior to starting his rivalry with Saint Shanks? Seems far from impossible to me.