*CRIES* Sorry, I'm a little tired, and when I'm tired I don't think straight. Yes, I confused it again, but I just edited it so it should be fixed. Also I'm a little stressed out, so that helps, and Pete's prank nearly killed me earlier.... *Babbles on*
Anyway, I am going to go back to my real self now....
Okay, if souls don't have anything to do with the dusk (NOT DAWN) then what was up with the Sora falling into darkness, after he stabbed himself in Hollow Bastion. I know this may appear to have nothing to do with it, but when you think about it it makes a lot of sense.
We know that the Dusk are forces of the Twighlight (As you explained) But they also appear to have a lot of symbols of the light (They are white for example, instead of black) which leads me to believe that they are forces of currupted light, or light that was currupted by the darkness. Now the Soul is, in essence, pure, but our hearts sometimes lead our souls astray (This is pure theory by the way). Now if the soul is pure light, then what happens when the misleading heart is taken away from it, and the soul is bathed into darkness (As Sora's was in KH1). My theory is that after becoming fully currupt, it becomes a Dusk, which as I said before, is currupted light, or in essence, a currupted soul. Now this dusk seeks to destroy the world, and turn it to darkness, or in symbolicism, make the day turn into night (Dusk).
Now the other side of the Twighlight force is the Dawn, who use the powers of Darkness and Light for good. These people seek to turn the evil into good, or in essence, turn the night into day (Dawn). This explains the whole Road to dawn thing, and also BHK, who uses the powers of Darkness and Light to fight the Dusk. This is just a theory by the way (But donations are still welcome!), but if you really think about it, it will start to make a lot of sense.