First of all, I'm really sorry this happened to you. It's strong of you to speak up and I don't think you are overreacting.
I think the drow scene as a whole really needs some double checking considering some of the characters that can end up in it - namely Shadowheart, Astarion and Halsin. It's not handled very well for any of them, especially since all of them have a history of SA.
When I had the talk with Halsin afterwards I was most of all annoyed that they gave him this background at all. It was totally unnecessary for his character and does not add anything to him. We already have two companions where this topic is handled way better. SA should not be used as a trope.
And I agree with you that his reaction made it feel like he partly enjoyed it (or maybe he's just deflecting). If you speak to him when he propositions you he can tell you about the many lovers he has had and one line is a fond "and I went to the Underdark, many many times". At that time I thought he had gone there willingly. This made his reveal later even worse.
Act 3 Halsin is a mess. I think they should cut this part from his backstory since it is not handled with the respect it deserves and because it does not add anything to his character.
