Only makes sense... If you ignore things she said that make her more monogamous than anything (which honestly seems like you have, read my post in page 45 for more detail). If they wanted to include a poly relationship in the game, it shouldn't have been with Shadowheart and instead they should have made a character that's explicit about it with no room for misinterpretation.
papercut was in the conversation before you made your account, making basically the same point as now. The dialogue choices pending on the player allow reading the character either way. It's up to the player to determine that. Not every player is going to agree with you, and that's okay. Imposing an opinion about a contextual reading on other players, or demanding that the ability to read the character that way by having the existing choices and interactions that support the read removed from those players for the sake of what amounts to a minor inconvenience due to a personally perceived inconsistency (which I will continue saying as not every player sees it as inconsistent, like myself) about choices that you sound like you'll never personally make is kinda just unnecessary. Cuz as I said, you can absolutely read her the way you're saying... if you don't pursue anything non-monogamous. And the ability to pursue that does absolutely currently exist and can be established as part of the relationship for a Tav that goes that route.
Sure it'd be nice to have even more characters with even more variety to them but as the game is it would've just been (and would still be) better to develop the concept more and not pin the lion's share of it on one redundant druid with some really toxic behavior. For all we know Shadowheart had things like the Mizora reaction and drow twins reactivity before Halsin was upgraded to be a companion, and if so therefor may always have been intended to at least be comfortable opening the relationship if a player pursues that. We have no way of knowing but given the dissonance between her interactions without Halsin compared to with Halsin I lean in the direction that Halsin was tacked onto something that already existed. But whether it exists or not should not affect how you read the character if you're not going to ever make those choices. The choices exist for the people that will make them. The writing could have been better, certainly. But I think we all just enjoy what IS there so much that we want perfection even when it isn't reasonable to expect it.
Speaking of misinterpreting, I personally didn't see anyone imply Shadowheart is okay with Tav cheating with Mizora because she is very clearly scolding you for it. She's being very forgiving of it, but certainly did not appreciate it being done behind her back. That's not being okay with it. But the scene does then serve to develop her character further based on the choice of having done it while in a Shadowheart romance. Whether a player accepts that development is again entirely personal to the individual who makes the choice leading to it and the people who most want these interactions to be gone were never going to make those choices to begin with anyway (unless they're lying on the internet), so their Shadowhearts remain entirely unaffected by them regardless, meaning there's not much of a compelling reason to damage the experience for the players that do make those choices.
Halsin is the exception to all this specifically because if he's in the party his behavior and reactivity to him circumvents all that, which is why Halsin (and reactivity to him) is the one in need of adjusting.
