I agree with pretty much everything you said, although I think it makes more sense for Ascended Astarion to be against Halsin, and Spawn Astarion to be a bit more permissive of him. Ascended Astarion is supposed to be very possessive and controlling and it's kinda strange that they're so inconsistent with it. Spawn Astarion's current reaction is alright as it is. With Shadowheart, yeah, Selune Shadowheart accepting poly and letting you cheat on her with Mizora is absurd. In act 1 and 2, she appears to be monogamous. There's that "I'd never want to be your spare lover" line like you mentioned and there's also monogamous aligned banter where she turns down Astarion for a date because she's dating you. What's even more bizarre, is in Act 3 you can tell her that she WILL be the spare lover in your relationship with Halsin, and that you're not sure if there will be space for her in your and Halsin's relationship, and she'll just be like "oh well there's a lot of places I can squeeze in between you two ;)". Like okay? I guess the romantic relationship is frivolous now that Halsin's arrived? All she wants is sex sex sex now? Truly awful and actually ruins her romance IMO. And that doesn't even touch on the fact that she flirts with him in Act 3 even when you're not in a poly relationship with all of them. Super tacky
Her being okay with Halsin and Mizora on her Sharran route is fine since she's not truly in a relationship with you anyway, although her reaction to Mizora still makes no sense. Why would you have to ask her permission for anything when her heart belongs to Shar and you're just sneaking around?
The biggest shame is that none of this will ever be changed. I think Larian is pretty much done making big narrative changes.
