Ok well to explain it simply: there is no (one) chosen one. Many people have, do, and will wield keyblades. Before KH1, there was a Keyblade War, so thats how we know that. Sora is just the chosen one to wield Terra's Keyblade (the previous owner). He is the chosen wielder of the Keyblade of Light, while Riku's is the Keyblade of Twilight, and Mickey's is the Keyblade of the Darkness. I think thats it.
Terra wasn't the owner for the KK, he simply chose who will wield it.
There is no Keyblade of Twilight, that goes by the reference of the Wielder, and even then it's using Darkness for the sake of Light, not any some sort of hybrid "element" in between. Twilight, Dawn and Dusk as elements don't exist. The 'Dawn' for Riku is symbolic, not "literal".
1.)Well first of al Riku was destined to be Keyblade Master (because Terra picked him) not Sora but because Riku's heart gave in to darkness (KH1) Sora got it bcuz he was the next strongest heart thus making Sora the Chosen One. We see Terra holding it in the KH2 Secret Ending
Ven factor might've had a part in this.
2.)Riku can wield the keyblade because he was already chosen to be master. Then he constructed the dark keyblade in Kh1 and then he got soul eater (i may have that confused :23
. And then somewhere along the way Soul Eater turned into Way to Dawn (KH2). We see Ven holding it in the KH2 Secret Ending
The Soul Eater's origins are unknown at the time. Most believe Maleficent helped Riku pull it out, using his innate wielding abilities in the process.
The Heart Unlocker made from the PoHs' Hearts was Xehanort's Heartless' doing.
3.) I don't know why Mickey can wield the keyblade. I know he got it sometime while in the Realm of Darkness and we see Aqua holding it in the KH2 Secret Ending
Mickey was a Wielder back in BBS before he ever set out to find the IKK. His original Keyblade was the Star Seeker.
4.) We also know Roxas can wield the keyblade because he is Sora's Nobody.
And Ven factor. Nomura said the reason Roxas and Sora can both wield at the same time has to do with BBS. Ven seems to be the likely factor for that.
5.) We know Xion can also weild the keyblade but the reason why hasn't been confirmed
Probably relation to Sora and Roxas. There's no way for her to have gotten the specific Keychain for the KK from anywhere else.
Last note -
Terra, Aqua and Ven picking up SRM's Keyblades is to show there's a bond between TAV and SRM, nothing more. Ven is obviously related more to Sora, yet he picked up the WtD. Aqua's ties are unknown.
The only thing we do know is that that movie kind of bases Kairi as not a Wielder in the present time due to the flowery Keyblade Riku let her use not being there.
