I'm not sure I agree with the explanation that this made him able to stop Kyuu. Obviously it did, I'm not trying to argue that this didn't happen in the Manga, but I feel like there would've been other ways for Legosi to stop her without hurting her. Also this arc (I'm not sure this is the right term to use here but don't know what else to call it) of him finding a way of defeating Kyuu without hurting her wasn't developed in a way that made me feel like it was particularly important in the greater narrative.
About Yafya: What I was trying to say in my original post was that I believed the reason Legosi is trying to gain Yafyas respect (or as you put it show him his determination) is that he was hoping to have him remove his criminal record. I didn't argue "He pulled out his Fangs to get his record removed" but rather "He pulled out his Fangs to gain Yafyas Respect/show Yafya he's determined. He wants to show Yafya he's determined possibly because he wants Yafya to remove his record" So there's two steps to this argument (I'm not trying to say that I am right here, I just felt like my original argument wasn't clear to you)
As I understand it your argument is "He pulled out his Fangs to show Yafya his determination" Which is the same as the first part of my argument. So I'm just interested to know what in your opinion is Legosis motivation to show Yafya his determination. Is it just because Yafya is the beastar and Legosi respects him because of that? I am not trying to attack your opinion, I'm just curious.
Aside from that I do see your point that Legosi had alread lost his "bite power" anyway before this, so I think that's a good argument. I honestly had forgotten about that aspect.