I look at it all this way: What if Sora really WAS the Chosen Weilder of the Keyblade? Terra said he PICKED Riku, but as KH1 proves, the Keyblade picks its master, not the other way around.
And judging by the Dive Into The Heart, which was before Riku used the darkness, I wonder if this means that Terra made a mistake, and while Riku had the ability to claim KK since Terra picked him, and Sora was meant to get the Keyblade from the beginning.
It even explains why Maleficent knew that Riku could summon the Keyblade. Since she apparently worked with Terra, he could have told her about Riku being his successor.
But its just an idea that I have and will wait to see if its true in BBS.