Riku is Twilight, he WAS Darkness, but he chose the path of Twilight at the end of CoM. Just because Mickey thought Darkness was evil during CoM doesn't mean he isn't Darkness. He might have realized that the powers he has aren't that bad now thanks to Riku. None the less it does state that he uses the Darkness Keyblade, why? Who knows.
As for BHK... I don't think BHK is part of the original plan, I don't think he was ment to be a Keyblader. He is fake, not part of the list personally. Given the two main theories, that he is Sora's Shell or A Shell with some of Sora's Heart, in the end he doesn't exist and chances are by the end of KH2 he will be dead / absorbed into Sora.