I don’t think that would count as “regressing”, really. His personal progress is minuscule and the only reason he’s not being a selfish bastard right now are Nao’s watchfull eye and her orders. So it’d be more like he’ll be unleashed again, which is why, yes, that’s what I’m expecting to happen if Nao loosens her grip on Yuu too soon for whatever reason.
I think the closest we’ve seen to the “true Otosaka Yuu” was in ep1, the “glorious escape” scene at the school gate. He’s escaping, grabs Yumi’s hand, she falls and he just runs away, leaving her behind.
From that, we can tell that he’s selfish and he can’t handle unexpected/extreme situations. That’s not a lot, considering that his selfishness is pretty obvious. In a situation where he senses a threat for himself, I’m expecting him to just cowardly run away. Note that in the scene I mentioned, there wasn’t even a physical threat to him. Merely his shenanigans getting exposed.
pls no Sartre, pls no Huis clos