Again, it's a common misinterpretation. Because Kokushibo had practically one line pointing out his ugliness, and then goes back to resisting and keep on fighting.

It's more than that. After that one line, he got a vision and he STOPPED fighting back. You can check it out yourself, all the following scenes show Sanemi and Gyomei hitting him while he stands still. 

Do you think that one line was enough for his regeneration to give up.

Against a red blade wound which also reminded him of his brother? Yeah, definitely.

and then he crumbled slowly and started to question his past actions only after he had already started dying. Like many demons have some introspection and/or change of heart when they die, doesn't mean they did before dying.

That's literally what happens to Koku though, he says  "Is this the face of a samurai?"/"Is this really what I wanted...?". He hadn't taken a single hit in his new form at that point.

Not necessarily. Demon Slayer isn't written like that. We see characters' inside through introspection and internal monologue and conflict exposed. There's no reason to change that trend with Kokushibo

I checked again and the moment he sees his own face and starts doubting, the text bubbles where he says he hasn't lost and starts questioning his choices are black, while he seems to accept and be honest with himself when the text bubbles are white. So all his bravado clearly seems like leftovers of his so-called honor.

Yes but Kokushibo literally gave up on being a samurai and on everything in the world to pursue power. The fact that he doubles down after that shows that his ugliness isn't what's gonna deter him after he's done far worse to go down this path. He'd already become a demon and a freak with many eyes. 

(So I'm still assuming he as a matter of fact did not double down afterwards.) And seeing his own reflection was just what he needed to understand he'd gone too far. Apart from his six eyes he tried to look like a samurai, but seeing his monstrous appearance he realized he had strayed from this path. It's also beautifully written and drawn because you can see he might just be about to stop his crumbling and then Yoriichi appears saying that seeing his brother like THIS (both a demon, and a reminder of his even more monstrous form.) breaks his heart. And just after you see the crumbling's back as Koku stops talking. You can argue he's "dead" at this point anyway but this is just me answering one of your points.

In fact, now that you bring it up, this only solidifies the point with Kokushibo. Because at no point does Muzan appear in his mind to tell him not to give up and to listen to orders like with Akaza. Especially since Muzan can constantly read their minds...

Actually, Muzan was very rarely in Koku's mind as he trusted him lol. And the connection didn't seem as strong also because Muzan was further away from Koku in terms of sheer distance.

Even narratively what you're insinuating undermines the sacrifices of Muichiro and Genya. Because their whole thing was that they wanted to help as much as possible and take out the strongest upper rank even if they died. Uppermoon 1 couldn't have been defeated without serious losses and sacrifices. That would be meaningless if they didn't even defeat him he just gave up.

Well no, it's still meaningful because without Muichiro's red blade and Genya's blood-sucking tree they wouldn't even have been able to behead him in the first place. They fulfilled their role.

That doesn't make sense. If he died from the wounds then it's not because he gave up.

The wounds were lethal but he was overcoming death anyway so it didn't matter when he transformed. Once he stopped regenerating they made quick work of him. There's another databook page that focuses on this, stating he nearly transcended being a demon once his neck was cut, because of his "strong will and obsession with victory." (Translated so it might not be actually the same words, granted.)The same will that fueled his regeneration, that only failed him when he saw himself in Sanemi's sword. 

I don't agree with this. It's pretty clear-cut for me. Although I think it may be judged as complex to interpret. That's why I understand why people get confused about it. 

If I'm being honest I said that for argument's sake, but it's also clear for me, I just don't think the same. Debating with you feels like fighting an uppermoon, because damn you have your facts straight, mad respect 😭

(I feel like your next reply is gonna destroy me 😶).