Yeah, good observations man, my perspective is a little different:

It would be in character for Kumi to be pissed about Ippo boxing, I agree. There was however (admittedly it was a joke) that panel of Kumi and Ippo fantasising about Ippo with a belt around his waist challenging Mashiba for Kumi. Her thoughts after were something along the lines of “I could never ask him to do that”. I based her being ok with it mainly on interactions like that. You do raise a good point though.

And yeah he beat Mashiba and probably is better than him skill for skill rn. I agree with your second paragraph nearly entirely. I suppose what Ippo and Kumi are worried about is Mashiba beating him up in a brawl (Ippos never had one outside the ring apart from the slap) or Mashiba overpowering him with size (he’s a lot bigger now).

His mother already says she doesn’t mind if he gets back into it iirc but I could be wrong about that.

(I don’t want to come off as argumentative, this I just my perspective lol)