Problem's with the theory is that BHK has a heart, as can be seen from the fact that he wields keyblades. Sora got his body back, Kairi's calling "pulled him together", per say. Plus, evidence from CoM shows that BHK has some connection to "the other side of Sora's heart", also indicating that he has a heart, and why Sora is "Incomplete". It could have something to do with Sora becoming a heartless, but BHK is certainly not a shell.
Plus, it's important to note that no-one actually identifies DiZ as a shell nor as the Superior- in fact, Riku mentions that he is "different" to the Order members. It may be that he is to Ansem what BHK is to Sora, yet he has accepted this, and might even be trying to speed the procces up by helping Riku.
Oh, and Dark Riku? BHK has been seen wielding the Kingdom Key also, so the situation is either simpler or more complex than you described.
And in my wake I leave questions