That Demandred and Semirhage chose with the Seanchan in Altara the same powerbase from what we know about the Forsaken and from their PoV. Demandred has to be somewhere else. Besides that, he tells in TGS that his rule is secure, but Tuon rules the Seanchan. Why wouldn't Demandred kill her and become Emperor? It also doesn't make sense that Demandred would work against the DF Suroth, but reward Tylee. Finally, the Forsaken usually don't impersonate known persons, but create new identities. It's much easier to avoid suspicion.