And you're basing this on what, exactly?
He is saying if you were take the keychain off the KK it would still be a keyblade. Which is why keychains don't matter. Even if it has a chain or not a keyblade is still a keyblade.
One represented Belle, one represented Beast, hence the different Keychains.
Actually both keyblades represent them both not just them separate.
That really depends on what you say the Ultima Weapon represents. I doesn't represent the Moogles, imo, and actually represents Sora himself - thus it makes perfect sense for it to change, because Sora himself changes.
We know nothing about the Ultima weapon, however, so we can't say this or there - only that it's possible.
Or they it represents an achievement in the game that isn't even needed to do to complete the game and decided to designa new keyblade for the game.