I want to say that I completely understand you as a player and as a human being, and I think such a change of Shadowheart's character for the sake of introducing polyamory into the game is completely unjustified. The fact that Shadowheart initially behaves like a character who prefers monogamy, is not going to participate in any “love triangles” in Act 1, and then suddenly makes such a sudden “U-turn”, it doesn't look like some “revealing a secret” or “new facet of the character”, it looks like the character was just taken and abruptly changed, without thinking about the logic and sequence of writing, or about the feelings of the players, who play in the game with immersion and are strongly emotionally attached to their LI. Why her specifically and not Lae'zel or Karlach? I think the most obvious reason is that Shadowheart is the most popular female companion for romance in the game, so polyamory was added that way. With Astarion, it's a similar situation, the only difference being that he clearly doesn't want it, and in his case Tav forces him into this affair by taking advantage of his affection. This “bear story” in general does not look good overall, in my opinion.
I think a mod that fixes this situation and makes the story better is a great solution. “It's not the way it was intended” - well, to be honest, the development of the game was pretty chaotic, a lot of things in EA were conceived much better than implemented (or changed) later. Even more so with Shadowheart's character, this behavior of hers with Halsin does look unnatural and forced. I myself play exclusively with mods that fix the flaws in the romance. Although the lack of the ability to play in co-op is certainly a problem, it's hard to advise anything here.
In Halsin's case, even roleplay for a monogamous player is not taken into account, there is no way to even refuse him properly, not only to disapprove, but even to politely explain what he is wrong about trying to invade someone other's couple. The option of saying you have a pair sounds like you're “fine with the offer”, which is completely inconsistent with what the player might be thinking about it. To the only appropriate option for refusal, a simple “No”, Halsin will get upset and say “Why did you ask me about love then?”, even if you had previously specifically ignored the possibility of asking him about his former lovers, just so you wouldn't have to talk to him about something like that. This just further illustrates how deliberate and unrealistic it's done, so I don't think it's real Shadowheart's behavior, it's not in keeping with her character, it's just that the developers decided to implement the polyamory theme in the game this way.
