It seems pretty unlikely that Demandred, the guy who wants to personally and by himself defeat and kill Rand/LTT, would go through the trouble of taking over a country just to turn over all control over it to another Forsaken. Besides, when the Seanchan arrived, there may or may not have been a king in place(I don't remember), but even if there was an official King, they had no control. Even when the supergirls were there, Andric didn't even have control over the capital, much less the country as a whole. It seems that Demandred is still not revealed in ToM, but I can pretty much guarantee that he wouldn't have taken a position which does nothing for the Shadow and provides him with absolutely no real power.