No, sorry you're thinking about a 'story'. This is an interactive story. Like DnD. Whatever you do happens and whatever you did last playthrough doesn't. The PFH showed the same scene but from 2 different perspectives. There's no one 'story'. You can kill Shadowheart on the beach and she's just a dead person with a strange artefact. The Dark Urge gameplay shows you can kill Gale. That's it. No orb. No Mystra. Presumably no game over too. What you're doing is metagaming here. All you know about these companions is what you find out through gameplay. You can tell Wyll to get lost and he's just some dude with a rapier and a big mouth. You can yeet Lazael in hell or let the tieflings kill her.
Honestly this is kind of like a dm mentioning an npc ooc and everybody in the party using that info in their rp.
The devs have specifically said if you do Wyll's starting quest and make a certain choice, you end up killing Karlach. As far as you're concerned, no demonbane, no Zariel, just a dead tiefling.
I don't think we can tell that yet - best example the strange artefact. Shadowheart had it for a reason. IIRC it was something she had with her and not from her time imprisioned, right? So people will be looking for that artefact. You might not know why she had it, but I doubt Larian will write permutations for every kill at different points in time.
I guess it can be simply solved by leaving details a unknown mystery to your character and finishing the main quest treating all companions like failed quests would end up in other epilogs. So I guess you are right
