Obviously most Forsaken considered that Sammael must be dead, but only Moridin and Graendal are really convinced (Moridin was in SL & has contact with the DO; Graendal looted Sammael's rooms after ACoS).

No one of the other Forsaken knows, though, that Graendal is such convinced that Sammael is dead. Therefore Graendal poses as Sammael to confuse the other Forsaken/create mistrust, because on the same time it seems then most likely to the Nae'blis that the only other male Forsaken alive, Demandred (except for Aran'gar), did this, and of course trying to weak or even kill Rand by this move.

Posing as Demandred is too risky and wouldn't confuse the others, to consider if Sammael might still be alive after all.

It makes no sense for Semirhage to pose as Sammael and send an army to Rand against Moridin's orders to capture Rand and against Demandred's desire to kill Rand personally.