I think this moment works fine even without romance or noblestalk conversation.
At this point in the story you walk into Shadowfell, where Shar holds the most power and you know that SH got direct orders from Shar to kill Nightsong. Without prior knowledge of the consequences of each choice it is not unreasonable to assume that sparing Nightsong would result in a similar outcome for SH as failing to rescue Mizora is for Wyll and as talking shit to Vlaakith is for you.
I think "good aligned" PC letting SH make the choice herself is not supposed to mean that you expect her to spare Nightsong, but basically saying "I don't like this, but I am not going to demand SH to sacrifice herself for the prisoner". And Shadowheart in turn surprises you by making a huge leap of faith.
