I see you going for the mistake that all of us have already done and try to make it cohesive and consistent somehow.
Save yourself the trouble, it's simply bad writing and scripting as a result of this being shoehorned in as an afterthought because they decided to include poly and swinger sex. It's just as inconsistent with Astarion and Gale. It was written as fan service for people who wanted to have a thing with Shadowheart and Halsin together (evident through the complete lack of foreshadowing in act 1 and 2 when everything else in her story is clearly signalled in previous events and the fact that non-romance Shadowheart and Halsin show literally no interest in each other) and they are the ones who will just have to suspend their belief to ignore all the inconsistencies when playing through it.
