If she was truly poly, she would agree on poly proposal in act2. That's the itchy part(In Gale dialogue Tav even asks her: "Why let monogamy get in our way" but she prefers that it does get in our way in the end, because she refuses.). Too much headcanon about "nesting partner" here to justify her answer. Mizora and Drows have nothing to do with being poly too. Halsin is the only question and, shockingly, thats the main reason why this topic was created in the first place(friendly reminder that Karlach and Astarion also let you do your thing with Halsin, but you can't have any poly relationship between those(SH,Astarion,Karlach), hence it supports idea that Halsin is just a fling). But hey, if you prefer to read it your way - do it.
