Don't need to explain the entirety of haki, we just need to get answers for a few questions:
The difference between regular CoA and CoA hardening, why is there a named and visual difference between the two even in the post-time skip?
Why are certain characters not using CoO in some instances, are they just not focused and as a result unable to use CoO?
Use your common sense, outside of the fans and Luffy, there is no one in the manga that calls COA
hardening. Not a single person. Someone pointed out that Koka is the same word for vulcanization (of rubber), and seeing as everyone else just says "Armament" (you will not find one person saying Koka or thinking it or even using the word "hardened" to refer to haki, not in the manga, anime, novel, filler, not even the games) and then their body turns black, it's not hard to add two and two.
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Vivre cards are irrelevant to me to be honest (and had directly contradicting info compared to the manga, see Shanks for example).
Lucci would have killed Luffy if he had any form of Haki.
Shanks was a mistake in formatting that was fixed just like how the manga has mistakes that are fixed in the volume.
How would Lucci have killed Luffy with Haki when Luffy mid-diffed two Haki users at once in the next arc and beat a Haki user in the previous arc? Make it make sense please. Inb4 but Luffy's neck stretched when Lucci shiganed it — yeah, and Smoker's smoke was still smoke when Luffy grabbed him with Haki.