How do you interpret that last panel with the "Haah..."? Could it be that Hina is getting tired of Ai's bullshit? Because I kinda am.
I see it as Hina being glad that Airi did come around as she had predicted, although it took longer than expected. She did start to doubt her prediction while they were walking into the ocean. She's probably suspecting that Airi is much closer to another breaking point than she thought.
Ah you mean it was more of a sigh of relief, I guess that could be it. Still, I'm also under the suspicion that Hina is also near her breaking point.
Both are good interpretations. No matter which one it is I think Hina will take the actions to save Airi.
If it's a sigh of relief, Hina will try to bring Hina back to normal life.
If it means Hina has had enough of it, I think she will make Airi totally her plaything so that Airi can never have any opinion about their future.
Either way, Hina won't let go of Airi.