No! Unless she proves she's loyal it's way too risky. She could just cut everyone's throat in their sleep for Shaar, or whatever twisted reason pops into her messed-up head.
Going by that logic, why accept Astarion into the party after his introduction is trying to trick you and putting a knife to your throat? Why not stake him when he tries to bite you in your sleep, clearly showing that he's fully intent on attacking someone in your camp?
Or why accept Wyll who's at the mercy of a devil and had already been sent to behead a different party member? He's unable to give details and clearly gets caught off-guard by fine-print, who's to say Mizora doesn't find another twisted reason why he must attack you or any of your party members?
And why not send away the walking time bomb Gale once you're aware of his condition and also that his usual magic item consuming is becoming less and less effective at keeping him from exploding? Later on he gets a mission to explode himself to defeat the Absolute, and not once is it on his own radar to consider the fact that exploding himself would mean exploding the entire group. Why would you trust him in this case? Even farther along he gets all googly-eyed for the Crown of Karsus like he's Gollum looking at The One Ring. Can you trust him to keep his priorities straight and not beeline to become the next Karsus at the expense of the group?
I'm not entirely sure how your personal story relates to the shadow-cursed lands, but I do know that it can apply to every single one of the companions in your camp. Who you consider "friend" or "classmate" in your camp will depend on personal preference, but there is plenty to be said why most of them are not trustworthy in some regards. Take it to the extreme though, and there's no one left in your camp and trying to do everything yourself is nigh impossible, so the mission will always be built on alliances with unusual bedfellows.
The issue isn't that she's repulsive. It's that she's such a contemporary character, it's easy to associate her with someone you dislike in real life - that's what happened to me. This is exactly why I don't support adding modern personalities into fantasy games.
So if you take the core background and personality defining parts of Shadowheart's character, someone snatched away at a very young age and indoctrinated by a cult, that's easy to associate with someone you dislike in real life? To me, it sounds more like for one reason or another, Shadowheart made you think of someone you know and do/did not like, the rest is more projecting those real life feelings on some pixels unrelated to the real life person she reminds you of. In that case, it's not really about wanting the companion writing to be done differently like in your original post, it's about the unfortunate outcome of mixing real life bad experiences too much with fictional work.
As for characters having contemporary aspects, regardless of whether or not Shadowheart or other companions are in fact written in such a way, it's often a necessary element to have a little bit of that to make a fictional being relatable and evoke emotion. It's difficult to make someone care one way or another about incomprehensible aliens, but (somewhat) ordinary peoples going through extraordinary times can make you relate to their struggles, triumphs, dilemmas, etc. in one way or another.
Leliana and Wynne (DaO), Serana (Skyrim), Neeshka and Elony (NwN2).
I can't say much on NWN2 as it's been too long ago for me to remember clearly, and unmodded Skyrim has been long ago for me as well, but what I do know for Skyrim is that pretty much all characters are super one-dimensional and faction loyal rather than PC character loyal, they have no individual goals that can differ from the PC and outside of picking their faction's side or the opposing side, they never involve themselves in PC character choices. From what I remember from Dawnguard and Serana, there is never a situation where she can have a differing goal from you and things need to be discussed, it's just having option/faction A or option/faction B here and there and her going along with whichever you choose. Because of that it's also impossible to be at odds with her in the way that can happen in choice based RPGs like BG3.
For DA:O with Wynne and Leliana , they both have certain deal-breakers that will make them fight you regardless of high/maxed approval rating with them, much like all the BG3 companions have.
When meeting Wynne when the crisis is going on at the Broken Circle, if you have Morrigan there and make the "wrong" dialogue option, Wynne will immediately become hostile upon concluding there's an apostate accompanying you. She won't allow conversation or explaining, she completely disregards continuing to protect the younger mages or your intent to help the Circle, she'll just try to kill you then and there with reckless abandon.
And at the Urn of Sacred Ashes, Wynne and Leliana will both become hostile when present upon defiling the ashes, high approval ratings have no influence on this. Just like with Shadowheart and her god's wishes, or Lae'zel and what she considers to be for the good of her people. All four of these characters have deal-breakers that go against their core beliefs and no matter how much they like you and how important you are to them, you can make a choice that will instantly turn them hostile.
Leliana has a niche case for being hardened and being able to intimidate her, but that isn't discussing things either, that's just threatening her into staying.
Karlach is a great companion but she's not human and not physically attractive. And after her emotional outburst following Gortash's death, I realized that either I kill her in act 1 (which first requires turning her into a freak with mods), or every single playthrough will end with me going to Avernus with her - which isn't great for roleplay.
So you don't find Karlach physically attractive, but to be able to make a different choice in a separate playthrough, you have to turn her into a freak with mods to justify killing her? Physical appearances aside, the entire idea about roleplaying the same story again but as a different character, is playing a character that has different morals than yourself and playing from their perspective rather than your own personal sensibilities.
Circling back to those physical appearances though, considering they seem to be a central theme upon which some moral choices are being justified for, why do you have to mod companions into freaks when you want to disagree with them and do bad things to them? What is it about their original appearances that requires you to mod their appearances to be able to make certain decisions when roleplaying the game?
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I explain why I dislike certain characters based on how they make me feel, but what's interesting is that no one explains why they like them based on their emotions.
I'm quite sure I've already done so in regards to Shadowheart, but I can re-iterate more fully. In my first playthrough, finding lore on Shar and Selune through books and other environmental markers early in act 1, it quickly became clear to me that Shadowheart was likely an indoctrinated victim of Shar's cult. Finding that her (dis)approval points often didn't align with Sharran doctrine affirmed this for me. Insight during her wolf memory, as well as connecting the dots from the earlier found lore, made it clear for me that she was an abducted Selunite as well. That put together made me feel compassion for a lost soul, essentially a frightened young adult clinging onto the only things her abusers wanted her to know and being guarded over her "truth" being challenged.
This made me want to treat her with patience and acceptance, allowing her to open up on her own terms and being able to voice her thoughts and speak her mind without immediate judgement or backlash. I quickly noticed how positive of an effect this had and on my first playthrough, which was good-aligned, her approval skyrocketed the fastest out of everyone. As long as I was gentle around the indoctrination from her cult and I encouraged her to share her doubts on her own terms, alongside her approval of my good-aligned choices, it took no time for her to become very agreeable and likable.
The date option became available long before the Tiefling party and I found it very endearing. She appreciated my slice of life memory when telling something about myself, I could reaffirm that she was more than what Shar allowed her to know by mentioning the smaller things about liking flowers and not being able to swim, then we topped it off with a kiss. From then on in travels the romance was sweet and the cracks in her faith became more and more apparent to the point of it becoming an actual dialogue option at camp. My initial assumption about her situation and the way I chose to handle the trauma brought onto her was having a positive effect and I was convinced she could overcome the indoctrination from Shar's cult and come out of it as her own, free-willed person. She had my trust. And in many dialogue options, she also let me know I had her trust and affection as well.
From that point on, it was a pretty straightforward baseline of trust, acceptance and a balance in how to handle things. It was us against whichever problem came our way and if she was ever faced with a dilemma related to Shar, then based on earlier experience and dialogue, I knew the only thing I had to do was give her space to come to the right conclusion on her own, only giving my own opinion when she specifically asked for it. She had shown the capability to think for herself and question Sharran doctrine before, so I trusted her to continue to do so and believed this was by far the best way for her to overcome her trauma and indoctrination.
After sparing Nightsong, breaking completely free from her abusers, there is understandably still a lot of pain and trauma, but the dynamic of trust and acceptance remains unchanged. And her goal becomes very relatable and important to me as well, saving loved ones from the cult she was kidnapped into. There are no more remaining conflicts or friction points either, she just wants to be with me whatever happens and we'll do whatever we can to help each other. Act 3's culminating romance scene is also wonderful calling back to her not being able to swim and wanting to feel she can accomplish things without Shar.
