I feel like the same things are just being re-hashed and dwelled for pages and pages.
What it all comes down to is that there is an option within the game to play out your relationship with Shadowheart as an open relationship. There are three instances where this can be opened up. One is through flirting with Halsin and then asking Shadowheart if she is ok with you having a fling with Halsin. The second one is by proposing to share her with the drow twins (and Halsin if he is present) and the third is by sleeping with Mizora (though I am not sure that one really establishes that you now have an open relationship, just that she won't throw your sorry ass out the door this time). Unfortunately the option to open up my relationship with her and other characters does not exist, but we can't always have everything. None of these options are forced upon you, though there are some uncomfortable and problematic situations with Halsin, but they are more related to his character than Shadowheart. And the dialogue tree in the drow scene sometimes seems to miss a step where Tav needs to propose this first, but that only leads as far as to Shadowheart saying that she is comfortable sharing if they are, and you can establish that you aren't.
I understand how this raises ideas and thoughts around her character, but I wouldn't overthink it. The writing that follows these paths and choices is just adapted and fitted to work within those "universes". For the drow scene to work it is simply necessary to provide Shadowheart with some enthusiasm for it. I don't think there is some deep character arc involved.
As much as you can find this inconsistent with the character, that's just what it is, something you feel plays out wrong in your story. Then just re-load and change your story. I can find it inconsistent that if I support Shadowheart towards the Dark Justiciar path I will automatically raise her approval so high that she will instead choose to abandon Shar and refuse to kill Nightsong if leave the choice up to her. That's inconsistent with how I've been playing the story, why would she do that if I've been actively supporting her to stay loyal to Shar and her desire to become a dark justiciar? Well, fortunately I can re-load and just intervene with a different choice in the dialogue and make her follow my intented story.
Likewise if I've been playing the story as an exclusive relationship with Shadowheart, I find my story inconsistent if all of a sudden she wants an open relationship. Again the game kindly gives me the option to keep my story going the way I want to by allowing me to choose other options and paths (aside from the unwelcome intrusion of a certain sexcreep that has issues with consent and boundaries). My story where Tav was in an exclusive lesbian relationship with Shadowheart (where I was a bit fortunate to make certain choices that lead to certain creepy men not trying to weasel their way in not being around) was very touching and appealing. But I don't find my story ruined because some other player decided to play a pansexual Tav that had an open relationship with Shadowheart.
Spending page after page just reiterating your thoughts around this and jumping down the throat on anyone who comes in and gives another opinion and perspective, does not seem productive or worth anyone's time really.
Halsin's writing and how he behaves in an unacceptable and very problematic way is a discussion that I think pertains more to him than Shadowheart, though she has a minor part in it with how the drow scene is scripted.
