Birth, you mis-understood MY theory ENTIRELY. Riku DID NOT always have THAT Keyblade. BUT THAT IS the Keyblade of the In-Between World, which IS The World That Never Was. There is a Keyblade for each world, and there are four worlds, Light, Darkness, In-Between, Present.
Somehow that Keyblade chose Riku, or Riku found it during hs travels in TWTNW. As for the Keyblade Graveyard, you should pay more attention. I believe it was the World Of Darkness, BEFORE it was locked behind the door of kingdom Hearts.
That world DID not always lay beyond the door. I feel that was the door BEFORE Darkness had fully warped it into what we see in the games. It's STILL the World Of Darkness, just not fully tainted. But the time of the games it had become entire consumed by Darkness, and probably also because of all the False Keyblades that lay in the world.
It's possible False Keyblades, Those Without Chains, are created from Darkness.
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And you do realize those three Keyblades are THE EXACT ones that Sora, Riku and Mickey have used. Those Keyblades Existed LONG BEFORE Sora, Mickey and Riku got ahole of them. Those may be their TRUE, first Masters, but I doubt it.
The Kingdom Key
The Inverted Key
The Way To Dawn Key
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Those always existed, and those ARE the ones the Knights pick up. And they are ALL NEW Characters. Not parts of someone or crap like that. ENTIRELY NEW. And one way or the other those events ARE in the Past.
And we all know about that Keyblade having No Keychain. That obviously means it's a false Keyblade, as the Oblivion in that ending didn't have a Keychain. Due to the fact that Nomura was hinting, No Keychain, Not a True Keyblade.
The ONLY True Keyblades are that of each world, Light, Darkness, In-Between, and Present. Nomura himself confirmed this LONG ago.