It doesn't matter if she's poly or not. If it were up to me, she'd be written to be disappointed and threaten to break up with you in the Halsin/drows (even without Halsin there)/Mizora situations. Because that's how she's been written otherwise.
However, player choice should matter. If you decide to not open up poly, she shouldn't do it for you. The interaction is free to stay if you accept Halsin and they don't change her reaction, in fact it'd be unfair if it didn't happen. But it should, like you said, never trigger if you decline Halsin. That's what I want removed first and foremost.
As it stands, she's not only inconsistently written to be okay with poly, but she is, if you bring Halsin along, also the one that actively initiates it even if you declined Halsin beforehand.
