Another point against: If Sethra is being so bloody careful to protect her alter ego, why send someone closely associated with her personally to deliver a message from her Alter Ego? Seems like a needless risk. On the other hand, how many people in the empire could identify Sethra Lavode upon seeing her. Of those people, how many could identify Tukko as her "servant". While I agree that it may not have been Tukko, I don't think the fact that Kiera was worried a button man for a low level Jhereg boss would recognize Tukko, and consequently, reveal her secret.