Remember Beakman: "Forgotten, but not Lost", Sora forgot Kairi, but she was still in heart.
Tho' you do pose a very scrumpious theory... they created BHK just like how they created the Riku Replica, hoping to get him to be on their side and open the door to Darkness.
And I guess they're having a hard time controlling him, it seems that at first he doesn't want to have anything to do with a keyblade or Kingdom Hearts, since he throws down the Keyblade and is fighting Axel.
BHK is so...awesome. He's like the punk rebel emo kid in a video game! LOL