That's where the roleplay comes in. In your playthrough, you didn't agree with Shadowheart saying that Nightsong's fate it hers to seal? You believe Shadowheart is facing an innocent victim and that she has no right to Nightsong's life? And you don't trust leaving it up to Shadowheart to come to the realization on her own that she doesn't have a right to Nightsong's life and that she should spare her? Then you interfere and come across the myriad of consequences depending on which options and follow-up options you choose. Then if she mentions stepping over your dead body if she has to, you can choose to fight or have two options to say it's a bad decision but to let her continue if she really wants to do that, or you can give a dismissive sort of whatever just do what you want then. If you don't fight, you get the continuation of the conversation between Nightsong and Shadowheart and all the different dialogue options that follow and can once again vary depending on approval levels.
Having seen the other options and their consequences in watching other people's playthroughs, I didn't find it that jarring there either to be honest. Of course, Shadowheart isn't being the most reasonable person in how she goes about things in the Shadowfell, but given that she's in the realm of a literal goddess who is demanding that she does something, it isn't all that strange that she's on edge and could react explosively regardless of approval levels. Many interactions with Shadowheart that have built up approval high enough to where it matters have given you opportunity to learn that Shadowheart eventually comes to the right conclusions if you give her space and trust, but also that she'll lash out quickly if she's feeling pressured or judged. So it's all about applying what you've learned previously and giving her that space in that moment, or going against what you've learned about her and pressuring her leading to her becoming aggressive.
For that single conversation option in the first dialogue tree, telling her not to do this, there's no variance between low approval and high approval. In getting to that higher approval threshold though, I learned that trying to tell Shadowheart to do something or saying to put herself above her god's demands would be the quickest way to turn her against me anyways, so these weren't good options to take in that moment. So what's left then is gently asking her whether she's sure and you can already see and hear the reluctance, and you have the option to trust her to make the right choice.
Personally, my biggest gripe in that moment with Nightsong is probably how the narrator seems to want to nudge you to telling Shadowheart what to do when you're on a high approval path, which is the exact opposite of what you've learned about her up to that point and which is also the exact thing you shouldn't do. That's probably what threw me off the most when first approaching the Nightsong.
And about the other companions not really reacting there also kinda makes sense to me to be honest, if you have three people shouting at each other, what use would another voice be at that point? They're there in your corner and hope things work out and stand ready to follow your lead whichever path you choose to take. It's also consistent with how they did companion reactions to other companion's important decisions as well throughout the game, there might be an approval or disapproval pop-up in the moment, but for the most part they'll only speak about the aftermath of a choice when asked rather than interjecting to give their opinion when the choice is being made. So in that respect I also didn't find it jarring compared to other bigger companion specific moments where the other companions watch from the sidelines like with allowing Wyll to behead Karlach or not upon first finding her. All other companions approve of stopping Wyll after speaking with Karlach, they all disapprove of fighting Karlach after talking with her, none will speak up until the aftermath of the choice that was made. I'd love to see everyone react more to everything, but the way it's done now is pretty consistent throughout the game, aside from an unfortunate drop-off in reactivity during act 3 that is very noticeable to me sadly, but the moment with the Nightsong didn't negatively stand out to me at that point in the game.
