Yes, but my main question is, can I go into the sewers to get Minsc without Orin killing the hostage?
Since he's close to Baal's temple.
Umm, he's not even close to the Bhaal temple? it's on the other side of the map. There is a waypoint right next to the door leading to the temple. If you open that door (and it's possible without doing any of the murder chain quests) that, and only that will trigger a timer in which she will indeed kill the hostage.
Also killing Sarevok triggers that timer.