About Kaido being injured, here are my thoughts:
Ryuo is known for penetrating strong defences. And it is a known fact that strong samurais are skilled Ryuo users.
Ie. someone like Vista wouldn't be better either.
Only top tiers can pull out casual Oden's slashes at Kaido which is understanding, considering their levels of CoA and their AP.
Someone like Akainu should have no issues penetrating through Kaido's dragon scales with his magmapunch + Haki.
That's why Kaido needs AdCoA as well to protect himself from such attacks. I doubt Kaido is that dumb not to learn anything from his fight with Oden. That was Kaido 20 years ago. Current Kaido should be vastly stronger than younger Kaido.
Either that, or Oda is really that incompetent and wastes the potential of a big arc boss such as the first Yonko being defeated by Luffy and Co. I hope Oda won't humiliate Kaido any further than that.
Furthermore, yeah, Oden sure is strong and so on but at the end, it is useless if the main antagonist on this arc is consistently used as a hypetool.
Imagine Doflamingo, Crocodile or fucking Katakuri getting this threatment. Hell, Katakuri outclassed his own mother on her own arc since he was the main antagonist of WCI.
It's time that Kaido shows what he is capable of - everything else would be really disappointing.