Interesting ideas there. Sounds pretty solid to me, although I would picture Tarkin as being in command of military projects and tactics, while Vader is in charge of the destruction of the Jedi. Actually, that would tie in nicely, as you could say that when Vader finishes off the last of the order, he returns to Tarkins side, on the newly constructed death star of course. I haven't put as much thought into this as you have though.

Oh, and I agree with you completely that Tarkin is the boss in ANH. He even calls Vader "My friend" just like the Emporer does in ROTJ... "You, my friend, are all that is left of their religion..." And he even "threatens" Vader: "This had better work." Can you imagine any of his underlings speaking to Vader like that?