As far as the Motti thing goes, I already said this before, but I think Vader definitely moves up in rank when the Death Star is destroyed, along with many of the high ranking Imperials. With power comes responsibility; In Vaders case the responsibility of choking anyone who screws up. Actually I just realized that in ESB one of Vaders first acts is to kill off one of his Admirals; something he was not allowed to do in ANH, assuming the above info about Motti is correct.