Yo, I've skimmed through the thread and read the opening post.
There is imo one major misunderstanding going on here.
Covering a weapon in haki is not automatically = advanced CoA 1/Shockwave/Barrier CoA. This way hardening your swords would mean, that characters are letting their haki flow -via an "advanced usage"- onto their weapons in order to then apply the inferior basic application. Which doesn't make much sense. And every average Joe without any CoA hype would therefore have advanced CoA, just because he once hardened his weapon. That's obviously not the case. Insert *random marines with hardened weapons pushing the birdcage/ that random BM pirate Kid with a hardened sword*
It's imo like that:
You exert haki around your body, that you can learn to compress to a tougher version of itself (hardening) or that you can let flow. What many characters in armed combat do, is letting bits of theirs flow onto their weapon and then go on to harden it, in order to strengthen their weapon. Cause hardening is obviously still the superior application in comparison to the normal "unmanipulated" state.
What you can also do is, to gather much bigger amounts from all around your body in specific spots, which you can then release/shoot out on contact(Barrier CoA), put into your slashes (Enma concept) or harden all of it up (Block Mochi),
That's imo what the barrier haki concept is and that is what Enma is doing with its wielders during those Enma slashes and that's what Zoro/Luffy were learning to control.
It's the advanced usage of the flowing concept, but the flowing concept on itself is not the advanced usage of CoA. You get me here? It actually makes a big difference.
That's how hardening is still superior to its "normal" usage, without any contradictions,. That's how not any random dude suddenly has advanced armament haki (like for example Alabasta Zoro) and that's how you can chose to not cut anything or even steel (let it stay in base form or compress it).
Perhaps the block mochi style is the way how Vergo muscled up his bamboo to such extremes and how he got that buffed look in the end against Law. This might explain it, cause we knew he was a CoA specialist afterall.
Edit: And the most advanced stage, where brawlers are finally getting lethal aswell, is obviously the penetration tech, ladv lvl 2.
Btw an alternative to the making of a black sword, other than "I hardened it for x amounts of battles" might be that you are using penetration haki onto your weapon, while you simultanously lock that ,normally very destructive energy, up via your own hardening.
Pre going to bed thoughts right here.