I don't see a reason to believe the key to returning hearts and the "keyblade of heart" are the same thing at all. Given how the powers of the keyblade tend to work it's much more likely IMO the key to return hearts is in essence somewhat of an abstract. That it's a power that can manifest through many forms be it the hearts of princesses, the keyblade, or just a person but only after the correct circumstances or a realization is made.
Well, here's why I believe it:
as I quoted earlier, in Report 6 Xehanort talks about the Keyblade of heart and says that only by gathering seven hearts of pure light one can forge a Keyblade of heart.
In KH1, Riku-Ansem's Keyblade that does a lot of stuff related to the heart is made of six hearts of pure light (and its design shows that it's definitely incomplete, since the teeth are broken and it's missing a keyholder).
They describe a Keyblade made of pure hearts, and I saw a Keyblade made of pure hearts. To me it stands to reason they're the same thing.
II'm curious to know the thinking behind this explanation. I'm not sure I understand why the six Princesses exposed to darkness at Hollow Bastion would have a problem but Kairi, who was directly exposed to the heart of Destiny Islands as it fell to darkness, would not? (I believe it's also implied or stated somewhere in the canon that Kairi was in the Secret Place at that time because she was holding back the darkness at DI herself until she became too weak to go on, but I can't recall where so maybe not.)
My interpretation is that it wasn't strictly Darkness exposure, but the fact that the six princesses remained at Hollow Bastion to contain the Darkness, and according to multiple lines of dialogue they were on the verge of exhaustion.
So methinks that they were actively doing something that might've sapped them of their Light powers, whereas Kairi sat that one out by escaping with Sora.
Bit off topic, but has Nomura/the story ever clarified the nature of the X-Blade? Like if it's light, dark, heart...I would think it's the last one given the manner in which it's forged (at least Xehanort's method in BBS).
Nothing extremely specific that I can remember, but I also think it's a Keyblade of heart. Xehanort does say that without a Keyblade of heart Kingdom Hearts would be unreachable.
However, in the same Report he says that the Keyblade of heart manifested only AFTER the world was reorganized after the Keyblade War... and the X-Blade definitely existed before the War.
More like, the War was fought for it. Unless Union changes stuff somehow...
? Is the Key forged from the PoH in KH1 meant to be the Key to Return Hearts? That would actually be kind of cool, to have it weaved back into the story like that, but I had certainly never made the connection if that's the case haha. It would also make Sora the perfect candidate for this ability since he's already experienced the effects of having his heart unlocked firsthand.
That's what I think at the very least. As I said above, connecting the keyblade of heart to Riku-Ansem's Keyblade only made sense to me as soon as I read the Report. Since KH1 the fact that Riku-Ansem had to say that his Keyblade was different and "unlike Sora's", bugged me (althought until BbS I just thought he meant that it was a Keyblade of Darkness).
Then when DDD came around and made a huge deal about unlocking heart as a Master level skill, I was even more convinced of my interpretation: Ansem needed that Keyblade because Master Xehanort was no more and couldn't provide that power.
Just like Xemnas needed the Kingdom Hearts to do a similar thing.
And yes, it would be kinda cool and a neat throwback.
