on the matter of Roxas's keyblades, it could be because since sora was asleep so the keyblade might have gotten roxas mistaken for sora, because he is his nobody. There are two because he has no heart, they came as his feelings for Kairi and Riku, since he had no heart to take a diffrent one. they are based on sora's feelings for them because his memory was being rebuilt, so he thought of the years he spent with riku and kairi.
For riku's keyblade, i think it might be a keybalde from a realm of it's own, because in the birth by sleep trailer there were 3 keyblades. The realm of light's keyblade, the realm of darkness's keyblade, and the way to the dawn. It also didnt appear (that we know of) until riku gained his form of light back, but could use the darkness.
I have positvely no idea about how mickey can weild the dark realms keyblade...
