But with Shadowheart nowadays, I just have her eat the Noblestalk mushroom in Act 1, and then I trust her to do what she feels she has to do. And she spares Nightsong every time. I do think that how you approach your interactions with her does affect the main story, like choosing respectful dialogue options that give her agency or making sure that the small interaction with the Noblestalk happens. That stuff directly leads to different story outcomes, and also new dialogue branches. If you do give her Noblestalk, restoring one of her memories, she expresses doubts about her faith multiple times in act 2, for example. This does not happen without the Noblestalk. So there is definitely an impact. Granted, this is not a romance specific interaction. But I feel like it’s the people who romance her who are most invested in her story, and most likely to make sure that the “secret” interactions happen?
I thought the memory about being attacked by wolves in the woods makes her question her actions when confronting the Nightsong. But that's besides the point, if you trust her to come to the right decision on her own, she most likely does, if you spent enough time with her before. If you try to convince her, to throw all her plans for the future away, you'll meet resistance. You could say what "breaks the immersion" is not trusting your (girl)friend.
Her decision in Act 3 seems to be far more random.
