Except...

Unless Demandred emptied the lands of DF, he can't really have a human army that will fight alonside Shadowspawn...

His Shadowspawn won't be mobile enough. From the north they can advance thanks to their overwhelming numbers... until they meet channellers... but the frontline to cover will be massive... In Murandy, the numbers can't be so big, the frontline would be much narrower and Demandred couldn't advance these forces into Andor nearly fast enough before Rand and Egwene allied to counter them (southern Andor is quite populated, especially on the border to Murandy. Pigeons fly fast), and any army of Rand-Egwene can pop up out of thin air, anywhere. The Shadow's channellers would escape this, but the rest of the Shadow's armies would have to stand and be mowed down, or be forced to retreat to the south-east and hunted down. To make things worse, there are the Seanchan who aren't DF blocking the only remaining pass to the South-west. The Seanchan are a spiky problem... without Semirhage there, the Shadow has to thread very carefully not to provoke an alliance with Rand because the Seanchan witness an overwhelming threat from the Shadow on their door. It wouldn't be so clever to turn Murandy into a large base of the Shadow. The Seanchan have their eyes very close on it at this point, and Tylee isn't the only pragmatic Seanchan officer. If the Seanchan rakens spot Shadowspawn in Murandy or get a whiff of channellers fighting there, Tuon will know in hours... and she'll attack. There's also the problem that as soon as this starts, one of the first targets Rand will strike at in Murandy would be the Waygate(s)... if there are even any - none have been mentionned, IRRC.

If Demandred has any involvement in Murandy, he would be far more clever to use it purely as a decoy/proxy or as bait in a trap but for this to work Demandred pretty much still would have be in control of the BT and the remaining BA too to strike when the baiting has worked.