Abdel Adrian was not the only Grand Duke of Baldur's Gate. There were three others.
Details are practically non-existent. Rise of Tiamat mentions that Baldur's Gate is still recovering from the events of that module but can still contribute the largest number of troops to fight the Cult of the Dragon. It does not state who replaced Adrian (though MiBG states that someone definitely did replace him) nor does it explicitly state who became Bhaal's chosen (though Torlin Silvershield is assumed to be the chosen should no one at the table have played MiBG) however, said chosen is alive and captured by Szass Tam rather than assumed to be dead.
When life turns it's back on you...sneak attack for extra damage.