Good post. I enjoyed reading this. I guess I'll go into why I think Roxas was a good idea in KH2 but poorly executed. With the Axel thing, what they could have done was have him kidnap and guard Kairi, bond with her while guarding her, then help her try to escape. This would have given Kairi more agency in KH2, while also keeping the 'do Nobodies really have hearts' story in but that falls in line with fanfic territory though. The Riku/Roxas fight is solid. I agree with you there in that he was important.
You would think Roxas would connect Sora to the Organization more, but other then the odd nickname they give him, Sora is not really that connected with them other then trying to stop their evil plans. The Organization calling Sora "Roxas" holds very little weight until after Saix is defeated near the very end of the game where he finally asks who Roxas is? Despite having many teases of Roxas through the game, Sora will use it to solve a problem, then largely ignores it (ironically the same way Roxas ignores his Sora dreams lol) even after they fight inside his heart it almost feels pointless from Sora's perspective because he wipes it from his mind shortly after to go save Kairi. The Final Mix version fixed that with the Roxas/Axel dream, but it still holds no weight on Sora in the game.
My biggest problem with how Roxas was in KH2 was that he didn't impact Sora's personal story, but they were building up to it like he was going to. They kept leaving clues and name dropping Roxas. They made Twilight Town and Hayner, Pence, and Olette real places and people when they could have just as easily kept them data exclusive. So naturally you think a big payoff is coming. But Sora and Roxas don't even talk to one another in KH2. Sora barely acknowledged Roxas after learning about him. The KH2 Manga actually fixed a lot of the Roxas/Sora problems I had because it did a better job connected them for that final act.
Roxas is my favorite character in the series now, so I'm glad he was introduced in KH2 and went on to keep appearing in games. Every game after KH2 I feel did a better job with him. Days gave me Roxas with his friends really showing how Nobodies can grow hearts to the point I was rooting for Roxas's team and against Riku's. ReCoded gave me a better version of the Roxas vs Sora showdown and how them communicating with one another really added layers to their relationship that KH2 lacked. And DDD finally gave Sora and Roxas the one-on-one conversation they had been lacking since Roxas's creation into the series. KH3 was just more of what DDD had to offer. It's because of these other games that I can go back and play KH2 and still enjoy Roxas there, despite all these nitpicks. The Roxas boss fight is even one of the best in the series for me.