I'm just saying that your assumption that:
The first line and the interaction with player after the failed attempt to kill us in sleep tell us, in a obvious way, that she clearly does not like betraying others.
It doesn't come as easy thing for her.
Is not necessrily true, because she might just be reluctant only because she likes you. If she didn't, she maybe could've done it without any hesitation, which one of the indicators is she does it without hesitation if you probe her for her negative thoughts, since then she is mad at you and no longer has a high opinion of you.
And that is why I have said before:
To be honest, I feel like Minthara features, as a character, a huge potential both to be reedemed or to be drawn into the darkness.
I honestly do not think, that she will be at any point good towards everyone, if redeemed. More like neutral and still uncaring, homever without harming innocents or others without any good reason. And certainly in very good relation towards PC.
I do not disagree with you about possible change happening with her, logically she could indeed become better overall, or just much better towards PC and much more calm towards others (selfishly though). Or much worse.
I think that this is well coorelated to your previous post; in other words, she will become more "tame", less violent, but will still remain selfish and uncaring towards everyone but PC.
We had more or less simliar thought, we've just expressed it differently.
I can see a reason for her to be even in party of good or neutral PC (when we join tieflings ofc, so before that slaughter, which makes her have way less blood on her hands), but this would require the Larian to implement the knockout mechanic and possiblity of capture.
I've said that in one of threads in feedback part of the forum, how could this be implemented. (Act 1 Druids vs Tieflings topic). If you are interested; here is the link:https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=748888#Post748888
Even when being evil and joining tieflings out of selfish reasons, PC could still make use of her, or just let her join you out of genuine sympathy/whatever you roleplay.
Same goes for Halsin still joining those who sided with Minthara. The more options, the better it gets.