An interesting theory, but not really huge.
However, I am wondering, who ever saw BHK Fight anything but Dusk, and Axel of course. That part of your theory could be true, however, it is obvious that the fact that he is the only one that fights the dusk is false, because many people saw sora get his *Shift*255 wooped by the dusk in the first trailer (Sorry, I hate to swear).
Also, THE RED HAIRED GIRL IS KAIRI, WHO BELONGS TO SORA!!!! Sorry, I am a huge Sora/Kairi fan, and if anthing gets in the way of that, it must be beaten (Riku....).
However, this new trailer which features BHK talking with Namine is interesting, because something could grow from that....
Other than that, most of your theory seems true, but not realy huge...