All of this might be related to the player character in Astarion's romance having a very fixed role to showcase his story.
If you want to pursue a romance with him, you cannot not fall for his theatrics in act 1. That scene in the woods is extremely fake, but to go on with the story, your character has to enjoy the performative act. There is no way to throw a wrench in by saying something genuine that forces him to work off script. You cannot tell him that he does not have to put it on quite so thickly because you like him, or that yes, actually you would be fine with just talking. You can't act as a person in that situation might, with sincerity or gentle humour to get to a more genuine side of him, your character is just there to showcase him in a certain way. In this case as a seducer. In the issue you all are describing in act 3, it seems to be much the same, you are there to showcase something.
