You're wrong with Astarion and Shadowheart though.

Shadowheart mentions in her quest, if(!) you loot a specific book, that relationships between other Shar worshippers were encouraged. That she liked that. There's an implication, that they kinda had orgies or at last changing partners. She mentions how she liked that about her upbringing.

It's not a cutscene. Maybe that's how it can be overlooked?

Astarion is okay(-ish) with sharing you with Mizora. It doesn't sound like he's too keen on it, either, and I think he could be poly, but is just too traumatized to really make an informed decision, but that's an entirely different can of worms. My point stands: Astarion does not only share with Halsin, he also shares with Mizora, so there is a pattern of him sharing Tav.

You're calling poly people selfish and emotional abusers, liars, and bring them towards hook up culture, which is very different from consensual non-monogamy. I don't like those pseudo-poly folks who are using the "I am in an open relationship" lie to fuck around without their partner finding out either!
They aren't poly, thought. They are cheater!

I think you're reading things in his behaviour that were not intended. Or maybe I am.

If Larian wants to clarify part of his romance, or adds additional scenes, I am all for it!