Ugh, the story that seems to show if you read this implications as you did is even darker and drow/halsin/mizora fits somewhat to her coping mechanisms…? Unconscious “I feel undeserving of Tav’s love so I’ll self sabotage” Like, when you meet
Nocturne in House of Grief, her best friend, she describes Shadowheart as caring, having a mouse once but ordered by Viconia to kill the animal in front of others as a lesson. Describes how they we’re together in a hideout making each other hair, very close to each other (could be read as a romantic relationship or platonic one). Even mentions how SH beat the crap out of transphobes there.
It shows SH as being soft and rather emotional there, not into casual sex or something.
They could technically rewrite it into that, but that's - as far as I know - how Astarion is written to some extent. I think he's okay with it so he won't lose you. But Astarion also wasn't very hard to get initially, he sleeps with you freely in act 1 already. And Gale can also be bullied into a 5-some with the Drow twins and Halsin, where he sits on the sideline as you get it on. Halsin in general just makes everything look bad.
If we were going by the self sabotage just to keep you narrative then she should give you approval for declining Halsin. As it stands, she initiates and encourages the drow twins hookup, disapproves of you wanting only her and not Halsin together with them, dreams of sleeping with him, and encourages a relationship with him. That's not self sabotage because she thinks she's undeserving of you.
As for the meeting with
Nocturne, it's not implied that they had a relationship past 'doing meaningless, small things that she'd forget even if she had her memories' together like talking and doing each other's hair. However, if you talk to Nocturne before confronting Viconia, she will say that 'if the rumors of what they did to you are true, I won't stop you from taking revenge.'. This is very vague and can mean just repeated memory wipes and sending her on the suicide mission to get the artifact, but if the Mizora chat was canon, the interpretation can be very dark indeed.
