Quote Originally Posted by Tavrobel
However, since Riku had already left Destiny Islands in the first game by the time the Keyblade got there, it chose Sora instead. When encountered at Hollow Bastion, Sora loses the Keyblade because his Heart was going through a moment of weakness, probably brought on by Riku's questioning of Sora's motives, and challenging his will by psychological means.

When Riku loses the Keyblade again, by that point, Sora had proven that Heart was truly as strong as it could be, and Riku had given to the Darkness, showing absolute weakness. Keyblade does not like Weakness, and goes back to Sora. Fortunately for Riku, he still had the Soul Eater in order to duel Sora with, which eventually becomes the Way to the Dawn (a Keyblade).
I would say that the keyblade chose Sora instead of Riku the first time because Riku had given into the darkness and showed that weakness, not because he wasn't there. He wanted so desperately to leave Destiny Island that he would turn to darkness as a means of leaving, and in doing that he lost him master standing. For the rest of KHI I agree with you.

If you look at the Soul Eater and the Way of the Dawn, does the Soul Eater have a keychain? Because if it doesn't (which I don't think it does), then doesn't the weapon he hold throughout the game have a keychain (which would make it the Way to the Dawn). If I'm wrong, then it's likely that when Ansem's machine blew up, and Riku was finally removed from the darkness, that his weapon may have become a keyblade, since he was finally worthy of one again.

It is possible that just like Sora (and Roxas) can hold two keyblades, Riku might be also have some means of holding two keyblades, and when he did he gave one to Kairi. Maybe....

What the keyblade is exactly has never really been explained. Xigbar however did mention that there have been those before Sora, so it's possible the there is a keyblade for every keyblade wielder, and if that's the case then there have been quite a few. Otherwise, one can only speculate.