The Murandian army won't know that they are fighting for the Shadow. Roedran will just announce that Andor has to be attacked and his army will follow. Roedran started to built his army in WH, exactly at the time when Demandred told "how busy he is". Besides that, it makes sense that Demandred was behnind the Dragonsowrn, who created lots of chaos in the Westlands, e.g. especially Masema.

Contrary to that, it makes not much sense for the story that Fain suddenly corrupts Murandy to get yet another power base, which will in the end of course be insignificant on its own, though.

Taim is most likely working on his own, just supervised by Moridin and SH. It's very unlikely that Demandred and Taim could work together, considering that they are very similar characters and almost equal in terms of OP strength. Plus, if that were the case, Taim or other DF AM would have told Demandred about Flinn & Co. Consequently, SH most likely gave Taim the Seal. Remember that the Shadow under Ishy already searched for the Seals in TGH and likely before (while Demandred started much later, as far as we know). Osan'gar was sent to the BT to support Taim or watch a bit what he does. But two Forsaken at the same place, namely Demandred and Osan'gar, doesn't make real sense.