I agree with all of this, but I would like the option for not to sleep with to not actually be until the end at the graveyard. You can even call him out in act one as him lying, so it seems a little odd that you just have to jump into bed with him. I personally head cannon that my Tav recognises that he needs it for control, and finds herself comfortable with giving him that space of feeling that.
I don't think Astrarion is asexual (he could be demi, but thats hard to tell through all the trauma). I think the question of an asexual romance is more about Tav being able to be demi or ace, which is an option not a single romance offers.
Halsin was my bet for it as well, and for the two first acts that seemed entirely the case. Then in act 3 he turned into a character I no longer recognised.