I think Hina's lacks the courage to come out and confess to the world that she likes him (people think they have nothing to do with each other). Whereas Rui actually hangs around with him normally in real life. Both of them have feelings for him but I think while Hina and Natsuo's love is two-way, its got so many logistical issues that they know their relationship will cause a divorce of their parents, Hina's sacking, the end of Natsuo's aspirations as an author due to the fallout and Rui & Hina blaming each other forever.
I know the feels are strong between Hina and Natsuo but I totally ship him with Rui or even Momo in comparison to him with Hina. She needs to be able to actually commit to the relationship and stop backtracking. Put it like this: Rui cares about Natsuo more than her image; Hina doesn't really. She can't even confess to him that she's leaving for that reason. And they both are really naive about hiding their relationship; Rui caught them and they didn't check the eyehole. And then they start snogging in the window on a school trip ???
What you said is true. but somehow I can't shake off the feeling that the author is going the direction of Natsuo and Hina being together at the end. somewhere along later into the manga natsuo might date with Rui or someone else, and i believe Rui plays a part in maturing him in a sense? Would look forward to Natsuo proposing to Hina again though. After both of them has matured of course!
Or alternatively, Natsuo might use this heartbroken experience to write a good novel story, one that wins a big award, helping him to move forward and ends up with Rui, who will be constantly supporting him behind his back? This might be another great ending as well!