@Didido Yeah that line seemed out of place to me too, so I re-read the raws. The line in question is きっと水嶋くんと付き合えたら幸せになれるんだろうな. The primary subject here is unspecified, so in a literal sense it says something along the lines of "Surely if X got together with Mizushima-kun, X could be happy, right?" X in this context could be either Kimura herself (the original translation), or Hiyama (something like "Surely if Hiyama and Mizushima got together, she could be happy [with him]?"). The two X's could be also different, e.g. "Surely if Hiyama and Mizushima got together, I could be happy [for her]." The current translation interpreted both subjects as Kimura, but I'm not sure I agree. Given how the previous lines are about how she feels her own love of Hiyama is selfish in comparison to Mizushima's love, I would think Hiyama is the subject, at least in the first clause. Its a bit ambiguous though, and I'm not a native speaker, so I'm not 100% sure.
@Dead-chan thoughts on this line on page 16?