Here's the thing.
[spoiler=In the secret video in Re:coded]
It is explained by Yen Sid that since Xehanort's heart was freed (by defeating Ansem, Seeker of Darkness in KH1) and since his body is freed (by defeating Xemnas in KH2), the original Master Xehanort will be free to walk once more. But there may be more than one of him.
He then tells Mickey to get Sora and Riku, to have them show the Mark of Mastery, which, as we all know, is what part of KH3D will be about.[/spoiler]
As for the rest, the Keyblade Sora has in KH1... belongs to Riku. It always was his, but once Sora had his MY FRIENDS ARE MY POWER speech in Hollow Bastion, the Keyblade switched to the one with the stronger heart. Riku also lost it because he was already giving in to the darkness.
This explains why both Sora and Riku have Keyblades. Another reason for this (and also why Kairi has one) is because in Birth by Sleep, Ventus, Terra, and Aqua (respectively) give them each the power to wield the Keyblade.
As for the light and dark keyblades... also explained in BbS is that there are separate families of Keyblades. Those of Light (which Sora wields), those of Darkness (same as the light but found in the realm of darkness; see Mickey's Kingdom Key D), and then those of Heart. That last one is only really relevant to the χ-blade.
Lastly, as to sealing the worlds... that's a standard feature to the keyblade. Locking and unlocking things, whether it's chests, hearts, or even worlds.