There's a very easy solution to this.
Never talk to the Warden. She will stay put in her room. Kill the patrolling guard and the two scrying eyes before they get to call for reinforcements (Thunderwave 1-shots them even on Tactician). Proceed to talk to Wulbren and give him any kind of blunt weapon (Quarterstaff will do), jump up the cliff in the back, run around and release the tieflings. They'll run to the boat and that's that. Nobody will be hostile afterward because they don't know you did it, even the Warden.
