Remember that the Shadow's plans for Caemlyn involved Elayne becoming queen; they spent a great deal of effort sending people like Hanlon and Duhara to try to control her, their Black Ajah mole never tried to assassinate her, and they had more than enough spies inside the city to know that Elayne's army could defend Caemlyn against Arymilla. So on the eve of an attack, Jaq Lounalt gets into Sylvase's good graces, someone who he expects will switch to Elayne's side after the defeat, to therefore get another inside man in the Palace.