@mirewitch
Dead-chan doesn't read japanese lol
since i'm the translator i'll explain my logic
It makes the most sense here for it to be Hiyama, yes, because Kimura is comparing herself to Mizushima about how well they get together, AND she's mostly talking about her own feelings where she kinda half-realizes that she's in love with Hiyama instead.
The reason I put it as it is now is because of the double potential form, which would make it literal
Surely if X was able to date Mizushima-kun, would they really be able to be happy...
The addition of that makes it more likely that Kimura is the subject in my eyes
(additionally, the だろうな seems to be questioning whether able to become happy which wouldn't even be a question if it was hiyama- that'd be more like, Surely if she dated Mizushima-kun, she'd be happy...)