I think RJ would have indeed prepared this twist at least a bit more in the books, not just in the Guide and in interviews.

Also Graendal says that the battlefield are the Westlands. All other Forsaken were active there. Therefore I doubt that Demandred has the Land of the Madmen or Shara under his thumb.

Besides that, if Demandred controlled so many channelers, he would have the resources the rescue Semirhage. But in TGS, prologue, he apparently doesn't have them.

In my view, the Shadow doesn't even need that many channelers, because the DO's growing powers will basically counter the Lightside armies pretty easily (turn weapons or armor into wax & water into blood, spoil food, randomly kill people or boost their "evil" character traits, maybe eventually blocking the access to the OP, etc.).

I still think the best solution is that Demandred controlled the Dragonsworn (especially Masema, maybe also those in Tarabon) and since PoD or WH additonally controls Murandy.