The guitar that reflected Satan's attack is covered in symbols and iconography from practically every major system of faith.
On that note, what's the action that actually hurts the devil? Not a rock off but a selfless act of sacrifice done out of brotherly love.
The D's use of an Invisible Back Up Band isn't immediately apparent, but they're presumably using The Power of Rock by playing the greatest song in the world.
In the climax, the D are challenged to defeat Satan in a rock-off to save themselves. After winning the challenge, more or less, they try to reconstruct the greatest song in the world, but can't. This is a rough dramatization of the D's song "Tribute," in which the D are challenged by a demon to play the greatest song in the world at the risk of their souls, then can't remember the song they played.
This is confirmed in the audio commentary.
JB offers KG to serve as Satan's sex slave in Hell if they lose the rock-off. This saves JB himself from damnation as well as prevents Satan from wreaking havoc on Earth, since it would require Satan to return to Hell.
It makes perfect sense that Satan will live in "The city of fallen angels," as Dio puts it.
Instead of praying to God for guidance like his father wants, JB prays to Dio. "Dio" is Italian for "God". Dio sending him on his mission to Hollywood is a blessing from God!
If Dio hadn't sent JB to Hollywood, then he wouldn't have met Kage and they wouldn't have sent the Devil back to Hell. They were truly on a divine mission all along. "Rock is not the Devil's work" indeed.
At the film's climax, the Devil was challenged to a rock off. Now one has to consider; who would judge this tournament? It would have to be God himself, since He is above all the contestants and would be (somewhat) impartial.
Another thing to consider; the challenge was a ROCK OFF. The Devil played metal, and then he tried to zap Tenacious D after they finished their portion. Of course God would send the Devil back to Hell after that!
Not that He wouldn't anyway, but at least this way the Devil doesn't hurt anyone first...
