... without first revealing that Rand is the Dragon Reborn, which they were unwilling to do at the time.

Nor am I certain they'd really care to assassinate Siuan. She was really posing no threat to the Shadow at that point, her fear of the Black Ajah was effectively crippling her. Her plan to bring Rand to the White Tower would have been fantastic for Ishamael - whether it's attempting to convert Rand by showing him that the Aes Sedai are keeping prisoner, keeping him away from his destiny by letting the Aes Sedai keep him on a leash, or killing/kidnapping him in the centre of the most efficient and powerful group of Darkfriends in the world. It was only Moiraine who managed to change Siuan's plans, and from there Siuan was basically out of the loop.

That's why I rather suspect it was Padan Fain who left someone behind to kill Rand.