Not sure what you mean here. If you mean save the inn by winning the Marcus fight then it is Isobel not Nightsong who has to be safe. Most people do this fight before getting to the Mausoleum so don't know who or what Nightsong is.
The approval rating with Shadowheart is only relevant if she is the one doing the dialogue with Nightsong. If she is not there the decision of what to do with Nightsong is yours (and it can be even if Shadowheart is present).
I'm not aware of any threat to the inn after Nightsong's release.
