How did finding Roxas a body even tie into the main quest if Roxas no longer had Ventus’s heart?
The implication was that there would have been no reason for [Roxas] to nurture a heart gradually the way someone like, say, Axel did,
I don't think it's necessary (or super on topic to the thread) to answer all of the questions you posed about Roxas, but I just wanted to focus on these two points above.
A constant theme of KH has, and likely will always continue to be, how anything and everything has the capability of growing its own unique heart no matter the conditions of their birth. And once something has its own heart it should be respected as their own unique individual person. This is something Sora very, very strongly advocates for even back in KH1 when he was fighting Riku over Pinnochio.
Even if you believe there was no "reason" for Roxas to grow his own heart if compared to a character like Axel, according to the lore he doesn't need one. Roxas became Roxas by nature of being...Roxas. Xion exists as Xion and not as a doll because she gained sentience and her own heart. The game claims data can have hearts too. Why wouldn't Roxas? Just because Ven's heart had influence over Roxas doesn't mean Roxas was forever incapable of being his own person. In fact, it would be extremely odd and incongruent to the lore for him NOT to have his own heart.
And the reason why getting Roxas a body can be tied into the main quest is simply because Sora wanted to save him. That's...literally it. Sora had a personal motivation to make sure Roxas could have his own self. It seems pretty straightforward to me. As Sora tells him in KH3D: "Roxas, you're you...you deserve as much as I do to be your own person."
From the events of Coded as well we learned that Sora was going to be the important key to saving all those who were lost (namely, Ven, Aqua, Terra, Xion, Axel and Roxas) which is what Mickey wrote in his letter to him at the end of KH2: "Sora, you are who you are because of those people, but they're hurting, and you're the only one who can end their sadness. They need you. It's possible that all your journeys so far have been preparing you for this great new task that's waiting for you."
So for Sora, his "main quest" was literally saving these people. Each of them. All shown as having their own "hurt" and struggles necessary to save them. Yes, Xehanort was doing crap and the war and all that, but he set out from the start to bring them back. And in every single one of those instances this quest was mentioned to Sora, Roxas was shown to be a separate person from Ven. So Nomura didn't just "bring Roxas back for the fans" or not explain Roxas properly-- Roxas' return has been telegraphed since literally Coded, with his unique identity being made very clear since Days. I don't know why you believed Ven and Roxas had the same heart, but many times in the game that has been shown to be not the case, and outright stated so by characters.
(as a side-note, when Sora finally "felt" Roxas' pain in KH3D, all of the memories that were shown were of Roxas', not Ven's experiences. Because the heart and experiences Roxas has as part of his identity are independent of Ven.)
If Roxas had a different heart from Ven, all the significance of that scene is lost and it makes no sense whatsoever.
"significance" is very subjective so I won't argue if you think it works better emotionally/narratively to believe Roxas = Ven, but saying that all significance is lost and its nonsensical otherwise is quite a bit of a stretch.
Many different people have "hosted" hearts within themselves, and often we see those hearts actively affecting their host and those around them. And while that influence can vary, it doesn't mean the host individual stops being themselves. In KH2, Roxas' unique heart affected Hayner, Pence and Olette through Sora when they ran to the train station to see him off. They had no idea WHY they wanted to do that (after all, to them Sora was just some weird kid in ill fitting clothes who burst into their secret spot) but they said they felt like they needed to. It's not because Sora made such a great impression on them, it's because some part of them felt Roxas' heart reaching out. After all, to Roxas they were his dear friends. From this experience, Sora wanted to become friends with the three of them, and then eventually did become friends with them in his own right.
You could extend the same reasoning to Axel and Roxas. Perhaps, yes, the reason why Axel felt an initial draw to try and befriend/get close to Roxas was because of Ven's influence, but that doesn't mean the only sensible implication is that Roxas IS Ven. It means that when people in the KH universe host hearts, those hearts and the connections they have with people can affect not just the host, but those the host interacts with. Axel even admits the reason why he sacrifices himself to help Sora in KH2 is because being around Sora made him feel like he was around Roxas again. Perhaps framing things in this way might help you see a significance to the scene you were talking about without denying Roxas autonomy?
More on topic to the thread though, I think sometimes people play the games, try to follow the lore and actually way over complicate things on their own. Sometimes even when the game outright SAYS things it feels like people don't trust it? Or assume the character who says it is wrong? Or ignore it? To be honest, it's probably because the series is very spread out across so many games over so many years it is very easy to forget things, misremember and then think the misremembering is fact. I'm sure I've been guilty of that now and again too since the older I get the less often I can go back and replay the games in my free time.
But it is a vastly different question to ask "is Roxas actually his own person?" (Because yes, he is, as stated by many characters many different times very explicitly to the point that I can't really understand how this gets confused) instead of asking something like "at what point did Roxas really start fully being his own person?" because the first shows a misunderstanding of the game lore and the other is a discussion on something the game is a little more ambiguous about.
Like, I don't have some crazy encyclopedic knowledge of these games. Sometimes someone asks a specific lore question and I'm like "oh yeah, I remember this being discussed in a BBS cutscene or something"--go pull up the game/game script/game cutscene, rewatch the scene for like 5 mins and boom the answer is literally right there.
Earlier in the Roxas discussion it was asked:
Why would the Organization even want to bring Roxas back?...if Roxas is a completely separate heart…why would they need HIM?
The cutscenes in KH3 in Twilight Town literally answer all the questions about Roxas that were posed here. In the scene where Xemnas and Seeker were talking to Sora they quite literally tell him yeah, you should go bring Roxas back because that's going to make you vulnerable to the darkness and we want that. It's not because they wanted Roxas, specifically, it's because they wanted a strong vessel to put their yucky horcrux darkness into.
This is just one example but it does underline for me the assumption that most of the "confusion" I see is because people just haven't played the games in a while, or skipped some games, or just straight up forgot. It's understandable to an extent, but it's just why I said earlier I get a tad frustrated at what I feel is an undue reputation of complexity when like 80% of the questions I see can be answered in a couple cutscenes.