I just can't see Kairi fighting...I just can't. Is like asking Aerith to kick ass like Tifa and give people pain. Kairi just looks too frail, girly, and she giggles. If she is to become a Keyblade wielder, she better make good use of it. And Riku-Keyblade, Nomura said that Kairi holding a keyblade is hinting something, not that she's going to be a wielder. It could be anything, but people will probably pick the most obvious. And the knights aren't Sora, Riku, or Kairi, or are tied to them, as far as I know.
As for the keyblade, if somebody other than its master holds it and it senses darkness, it automatically goes back to the owner. At least that is what I saw. Kairi is a Princess Of Heart, she has no darkness. It has nothing to run away from. We know that Riku gave it to her, but how he got/made it, we don't know.
We've only seen Kairi used it for awhile and it disappears and never comes back. Kairi didn't summon it back either. When their group (Sora, Riku, and Mickey) used their keyblades to open that door in TWTNW, Kairi wasn't with them. If she was the next wielder of the Present World, or what you think it is, she would have played a part, wouldn't you think?
Nomura has to pull something good out of his head if she is to become a keyblader, cause that there is a plot hole.
The only one that was really chosen, IMO, is Sora due to the Awakening. Riku made his Way To Dawn (we don't know if he had a dream), and Mickey went to the realm of darkness to get the other Kingdom Key.