I guess there's a possibility Mat makes the decision on his own after witnessing the loss of Caemlyn to the Shadow (nice to see this confirmed at last, that's been a pet theory for several books now...) and upon hearing the WT did get attacked by the Seanchan. Mat is a military genius and he's going to see this in military terms (and he's got Moirane and Thom to discuss the politics with him). It's far from impossible he coldly calculates the Light has either lost without an alliance with Tuon, or far more interestingly, that the only way to avoid the Seanchan clashing with the Light, he must convince the Empress to give her Prince of Ravens the command of her armies for Tarmon Gai'don (in a mirror/parallel of Barid Bel Medar's betrayal... Mat would do this to serve the Light, but that wouldn't sit well with Egwene... possibly not even with Rand though that's less sure. I guess this could be the whole "I would never betray you" comment from Mat's PS trip. Not the Shadow... Mat resurfacing at the head of the Seanchan armies when the Seanchan are still enemies. And it's Mat who will have these armies fight the Shadow, not the Light. I can even see one obvious way he might do that.. by demonstrating to the Seanchan generals and Tuon their planned attack on Caemlyn will totally fail, and how he'd lead that battle... knowing all along but not telling it's now the Shadow the Seanchan will face there. That's how his reputation among generals grew originally, and how he found himself playing key strategic roles. Mat can be as crafty as a fox. If he can convince Tuon of his worth, and she's all but figured it out on her own already, he might manage to deceive her.) That would be a way in wich the alliance between the Seanchan and the rest only comes once the LB is well under way and the strength of the Shadow leaves them no other options but to unite. By then, Mat would probably be as popular and revered among the Seanchan soldiers and officers than he is (soon, was?) with the Band... perhaps more than Tuon herself...