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.
The biggest shame is that none of this will ever be changed. I think Larian is pretty much done making big narrative changes.
I agree with this. There were moments in the game that fell short of showing AA's more possessive, controlling or protective tendencies. As it is it's sometimes inconsistent. Not likely to be changed much now though. -.-
Personally, I see it differently.
If AA has no problem with TAV having a fling with Mizora, it seems logical to me that the same thing happens with Halsin. For AA, I suppose things are on the same level: both Halsin and Mizora are a distraction, a moment of fun. AA's possessiveness is not in not allowing TAV to do what they likes (see also allowing them to follow Karlach into Avernus), but in not allowing them to leave him. He allows TAV to do what they wants, but the deal is that they must stay with him.
