I see where you're coming from mentioning Leliana's situation (as well as Wynne and Alistair from what I remember?). However, despite me not knowing about Nightsong points and all that in my first playthrough, all the signs for Shadowheart's personal struggle were there to find in my experience. They were even neatly put down in her personal quest journal to read back in case I missed something in the moment. And the choice is phrased somewhat differently than what you say and the nuance is a part of what swayed me personally. The choice for the first dialogue round says "Trust Shadowheart - do not interfere". Given previous dialogues with Shadowheart, the talk that she's having a crisis of faith, the talk about our relationship and that she seems distracted and drifting a bit in act 2, as well as the high approval I had with her, I did trust her. For me, I had seen enough signs leading up to that decision to believe that she would end up sparing Nightsong all on her own.
It's totally fair if you do not feel that way, many people did feel they had to interfere with varying results, but many people also did choose to trust Shadowheart in their first playthrough. And if you want to roleplay standing for justice, like a paladin with a specific oath, a righteous monk or a cleric of Selune for example, it can definitely make sense in that moment to not trust Shadowheart's ability to make the right decision and end up fighting her over this. If I were to put it in D&D terms, Lawful Good would be more likely to want to take charge and interfere I imagine, Chaotic Good could be more likely to trust their friend to do the right thing without them having to interject themselves.
As for the writing there being insufficient, agree to disagree I guess. All I can say is that if you're interested on the matter of getting people out of an indoctrinated mindset, there are plenty of documentaries on people who have been in a cult and/or people with Stockholm Syndrome after having been kidnapped for a certain amount of time. Directly pushing against what they've come to firmly believe in their time being under someone else's thumb is usually the quickest way to get them to lash out and become violent in one way or another. Giving them space to breathe and gently supporting them in starting to think for themselves and weaning them off of the indoctrination they were led to believe as truth is a healthier approach to get them feeling better and more like themselves again. So in that light I personally don't feel like the writing for Shadowheart in those moments is insufficient and I can see certain decisions leading to certain results given those real life examples.
