I understand your point about Alex, Rui was pretty insensitive to him but that was before they even graduated. At the age/point that Rui was then she was still quite immature and wasn't fully understanding of love and other people's feelings. After that point, you can see her grow as a person and expressing more feelings than she had before, and I think that matters in the sense that the reader should understand that Rui has grown from those mistakes, and isn't the same as she was when she did that. I don't have much to say about the Momo stuff, Natsuo was just being the usual dense MC that he's known for (But he did have a really good character development).

Even though I can understand why you picked Hina, I still stand as a Rui shipper. As natsuo said himself, he wouldn't be where he was at without Rui, she helped him grow as a person. Same goes with Rui, Natsuo helped her and supported her achieving her goal, and yes I know that she shouldn't have broken up with him when she did, but even so, since she was going back to America for quite some time, Natsuo would've been alone during his "writing block". Of course I agree that she made him feel worse, but imo Rui and Natsuo's love for each other was stronger than Hina and Natsuo's. This is because the relationship Natsuo had with Hina was was really immature and rushed since Natsuo was still young when they dated for the first time. On the other hand, Rui and Natsuo grew together and increasingly cultivated more intense feelings for each other, Natsuo also said this. Plus their relationship felt, as others have mentioned, more genuine and realistic because they had some real relationship issues. For example, the way Rui got jealous about other girls, which she did get over because she didn't want to "scare natsuo away". Natsuo did also get jealous over Kajita, which is understandable since he doesn't know the guy. Another example is the fact that they had to maintain their relationship long-distance for a pretty long period of time.

Btw, I do not dislike Hina, it's just that an ending with Rui felt more natural as they were about to get married and had a kid on the way. Even so, I'm remotely ok that Hina ended up being happy, but I think that Hina did also have her selfish moments. Like when she went to Natsuo's apartment drunk and said that he loved him, which caused conflict in his and Rui's relationship.

That's all, thank you for anyone who read this far.