Things I've personally noticed after replaying and trying different things. 1. About the twins without Halsin, okay, it's not uncommon for couples to try different things sometimes. I can get that, and I don't think it's way out of character, it is more like, she's checking if you're okay with it. I can accept that from her tbh.
2. With Halsin, all I can think it's that there is a bug in the flags because let's be honest here, Act 3 is a nightmare to any player who wants immersion and wants characters to stay in their place. I got flirts in Act 3 from characters I had said no to their advances, I had random lines from Gale, Karlach, etc. All acting as if we were a thing here and there. So, for now, at least, I will just accept and hope it's a bug.
3. About Mizora, I never had any interest in her so I had no idea, and I went to yt to check the video after learning about it. And I will be honest, She is not okay with it. She may be saying that, but the lines, the way the VA portrays Shadowheart? If you ever were in a real relationship you'll know, she may be saying she's ok, but she's not. And here it's you to blame, you cheated on her, and there's nothing you can use or say as an excuse to justify it, even if it's a poly romance with her, it's still cheating on your partner. The way she talks about purity, etc. She's just mad at you and realized that to have you she'll need to accept that.
I agree with most of what you said (I don't think its a bug tho, since bugs don't suddenly materialize words into existence. It's obvious that the writer intended for the Shadowheart/Halsin thing to happen during her romance) but those scenes aren't the issue on their own, its when you put them all together is where the huge Halsin shaped problem pops up.
If the MC sleeps with Mizora it is obvious that Shadowheart isn't happy and that the MC cheated, but her contradictory line about her not looking for anything pure or loyal (her not being more upset or even breaking up with the MC) is what paves a path for Halsin. Add the banter to that and voilŕ, you have the shit show that everyone is complaining about.
About the bug, I mean the fact not of it being in her lines, but maybe it's supposed to only trigger if you're romancing both. It's what I hope at least, time will tell when they release the finished game in a definitive edition. Until then, I'll just let it slide as a bug personally.
About purity in her line, if you pay attention, it's where she even raises her voice more, it's because she's mad, and can be also seen as if she will accept it because she doesn't want to lose you. And I also agree that is because like was said by others and it's already established, they removed ALL consequences in Act 3. So, there's that as well, you can basically do whatever and things will just work out to basically the same exact path.
