Kool. I can see where you see the IV chord as a subtonic in terms of chords. But you may what to watch because a tri-tone can exist in a IV chord as well (#4). The subdominant name given to that chord is just music terms in the least.
Chris-
About the bVII chord in a progression. I'm not into analyzing or even listening to much gospel music. But a common chord progression coming and going from the bVII chord is: bVI - bVII - I. This one is a good build up to the I chord. Think of this one in terms of it's roman numerals because it doesn't have the same effect if the last chord isn't a major tonic chord. The bVII as a suspension usually has to come from and return to the I chord. You probably ran into the bVI - bVII - I quite alot, it's a awesome progression from the norm.
"My whole life is a dark room...ONE BIG DARK ROOM" - a.f.i.