Hardening = making whatever you’re coating harder. It does not just make skin harder it makes anything harder.
What are you even talking about that advanced haki just bypasses haki, that’s not a thing.
Exactly, on body it's just harder/thicker skin. On weapons, it's harder weapons. Etc
Kaido's scales is super hard by default, hardening is irrelevant to him.
Internal destruction variant of advanced armament does. Luffy explained in Udon that his aim was to bypass Kaido's scale to begin with.
Advanced Conqueror's is simply strong enough to damage the dragon scale itself, and implication anyway was that dragon scales could by significantly damaged because advanced Conqueror's was "deep" enough.
Advanced Conquerors eat hardening for breakfast, it's simply not relevant. You can defend against that the best using your own advanced Conqueror's, or strong barrier advanced armament.
Let me ask you this. If hardening was even remotely relevant, how come pre udon Luffy did zero damage to Kaido? How come Kid still can't damage kaido without crush?