The problem with this is unless Demandred uses Travelling openly in front of the Murandians, his armies advancing in Andor will be spotted long, long before he comes anywhere close to Caemlyn (jut like the presence of the Borderlanders outside Andor was spotted and reported), and Elayne can and will use Travelling to send forces ahead to meet him in battle.

Talmanes's POV really gave the impression there's nothing much left standing in Caemlyn at this point - even the palace was about to be put to the torch and the rest already is. I agree the Shadowspawn army has to be dealt with, it's a major danger to villages, towns and the other cities of Andor, but Caemlyn is seemingly a goner. And it's soon to raise questions as to why the BT didn't get involved. Elayne might want to retake Caemlyn for the principle of the thing, but all her entourage will try to convince her it's madness and she should focus on saving and relocating the population and let channelers deal with the Shadowspawn. Assuming the Shadowspawn will actually be at Caemlyn and not all over the countryside (less likely: gone through the Ways) when Elayne finally sends a messenger to the city.

Another possibility is that Taim will come out of the BT and raze the city, pretending to have destroyed the Shadowspawn and buying himself a few more weeks of pretense the BT is serving the Light. I wouldn't rule out Taim as "savior" of Talmanes (if Talmanes isn't the character Brandon had surviving that Harriet forced him to kill. He spoke that he had this character live through the whole book... it could mean someone who's wounded early...)

I guess Roedran announcing he's secured an alliance with the BT to fight the Seanchan is yet another possibility... prolonging the pretense (to Murandians) they're serving the Light in this. And this could still bring about a clash of the Light in Murandy. Rand's forces would know about Taim, but not that Mat and the Seanchan are also attacking.

After TOM I had this notion that the Seanchan were about to strike the BT and would find the Shadow attacking Caemlyn and the forces of the Light already fighting and it's how the big clash would occur. I put that aside once learning Mat joined Tuon, and so there must be time now for him to be put in charge of the armies, meaning any clash is likely to happen later than I thought, and the Egeanin scene also suggests this - Egwene has to have time to hatch a plan for the damane/sul'dam (which isn't such a good thing on the surface of it: Tuon's and Mat's control of the imperial armies could start to unravel in the middle of the LB). This is one "devious" way Mat could force Tuon's hand: he would agree to lead her armies against Rand and co., but once he has them he'll actually use them to attack the Shadow. It rested much on Mat's knowledge that Caemlyn had fallen to the Shadow... on the pretense of leading an attack on the BT, he would bring the Seanchan armies where their generals would witness the LB already ongoing. And now this scenario doesn't seem likely as Mat appears in chapter 11 already in Ebou Dar. The leaders at Merrilor will probably learn from Moraine he's gone to convince his wife to make an alliance.. only to see days are passing and Mat is not returning. It's gonna take him a long time to convince Tuon of anything, much longer than he hoped, and he might have to deceive to pretend he's siding with Seanchan views. It's by getting control of the armies he can bring the Seanchan to fight on the right side, if Fortuona proves too stubborn. That's presumably his apparent "betrayal" of Rand, IMHO: for a while Rand's side will assume Mat has joined the Seanchan for real.