Shadowheart does suffer from being quite difficult and unpredictable to read and her personality being all over the place.
This situation is complicated by the fact that in the previous scene where she goes after Lae´zel, you are supposed to talk her out of it, or she goes through with it and kills Lae´zel. So based on previous experience, this would encourage you to try to tell her not to go through with killing Nightsong. But all of a sudden, that's the wrong approach and you are supposed to do the opposite and wait for her to make the decison on her own. How am I supposed to know this?
My own experience when playing the game the first time and not spoiling anything in advance was that I simply had a Tav that was a charisma skill monkey and just passed the skill check to talk her out of it, so that wasn't a problem.
