Yea, that's how it played out in my observations as well.
It started with Blank Points in BBS more or less, but typical Nomura-like it was kept highly ambiguous and several possible outcomes could be interpreted from it for all involved characters presented there.
I remember the forums here being filled with debates back in 2011 and following years over which route would be taken from there.
Then came Coded and Re:Coded with the whole scenario around Data-Sora and the question if a heart can be born/created in a being of data and possibly also elsewhere as well as the final part of the game in Castle Oblivion which connected this theme further as well as being connected to Blank Points as well.
The whole theme of "non-natural-born" beings capable of having their own hearts was at the same time also a throwback to Chain of Memories where Sora already insisted that the Riku Replica, despite not being Riku still had a heart of his own.
It was also in the "new" secret ending of Re: Coded where Yen Sid and Mickey debated about the whole procedure of beating both the Nobody and corresponding Heartless will bring back the original person, something already mentioned in the KH II Final Mix-Ultimania years before but was only made 100% canon by being included in an actual primary medium of the series.
Back then there were also countless debates if and how that would apply to Roxas, Naminé, Xion and Xemnas because they were all unusual special cases.
This whole narrative string came then to a culmination in DDD where the question if anything can have or gain a heart of their own was ultimately answered with yes by both the "good" side via Ansem the Wise and the "bad" side by both Xemnas and Xigbar.
What exactly that meant for characters like Naminé, Xion and Roxas was still kept ambiguous even then although the whole "they can form and nurture their own hearts" already pointed to them being their own persons in essence.
Yet it was Joshua from TWEWY who spilled out the essential conditions to if a being would/could be considered their own individual:
And Nomura played with this during Sora's final world when he finally met Roxas face-to-face without it escalating into a battle and by throwing another curveball.
Originally Roxas was arguably the character who fought for his own identity and individuality with the most effort and fervor seen in the series, but during the sequence there in DDD he seemed to have given up and accepted himself as "just a part of Sora" and it is Sora himself who immediately interjects and rejects this reading, insisting that Roxas has the right to be his own person and that he sees him as his own person as well.
I may lean out of the window here but I do think that this scenario was still part of Nomura's indecisiveness on the matter as it was still open as to which view on things would ultimately prevail and happen in KH III.
Roxas' by now established view that he's "just" a part of Sora or Sora's seeing Roxas as his own individual.
Lea/Axel and other characters who do see Roxas as his own person would certainly also play a role towards this according to Joshua's explanation.
With Naminé, things were more subtle and less shown, but there was another promise between her and Sora beyond the whole "Thank you"-thing and that was Sora promising they'll become real friends once they meet again in person.
Kairi having thoughts about Naminé in III regarding her status and possible existence of her own heart and thus being a person of her own were a nice groundwork for her possibly starting to grow a bit as a character and not being always fixated only on Sora but sadly it didn't amount to much more.
All in all, things were however subtly developed towards them being their own persons, with Xion obviously being the most complicated case because of all the memory shenanigans and I think I do remember Nomura himself stating either in the Coded- or in the DDD-Ultimania that Xion's "return" or similar would be the most difficult to achieve.
All in all, the whole issue seems to me like a repeat of the whole earlier fandom-splitting question if Nobodies really have no hearts or rather if they remain constantly in that state.
There were several competing interpretations of this issue ever since the end of KH II and the one that they can somehow regain a heart won out in the end.
