When reading about the Meeting with the Forsaken, I got an idea. What if Demandred was told he is Nae'blis by the DO (part of the scheme mentioned in LoC) but publicly Moridin was titled Nae'blis. I completely believe a double cross by the DO and Moridin is Nae'blis, but orders come through Demandred (and he believes he is) and then given to Moridin. Totally unsubstantiated and no evidence besides a crazy idea, but Dem's answer about his rule is secure made me think of that. Though his "gathering for war" comment could just be that he is in charge of the trollocs gathering across the Blight.