I think I can somewhat answer the Kama and Parvati bit, I think it's likely the resentment about her sealing away Sakura's darker side away, while I get the idea behind it, it's essentially "sweeping the problem under the rug" type of action while not accepting both the pretty AND ugly side of someone. Yes her being the wife of Shiva is a factor, it's more so Kama being the god of love, requires them to understand humanity more, BOTH good AND bad, not a personal level but more like a desk job type of deal. So the fact that Parvati just "shut away the unsightly side of host" Kama took it as a chance to show her up and point out her own flaws seeing as she was so quick to hide away that side of her, which is something that only Parvati has done out of all the pseudo servants. Not one has done what Parvati has done, at least not so overtly. Kama chose Dark Sakura, because she was "REJECTED" by the "GOOD GUYS"
I don’t entirely agree.
I feel like that'd be a legitimate factor if Kama seemed to care about Sakura or that part of her, but she doesn't. In all actuality, her treatment of Sakura is far shallower than Parvati's, considering the way that she switches around Sakura's body to fall into some line of 'appeal' like some scuffed doll. Even reducing Sakura's trauma-infused childhood body into something 'cute'. Kama doesn't care about Sakura in the slightest, if anything Sakura's influence is what's giving her a smidgen of humanity that Kama is consistently baffled at rather than accepting towards. There are moments where Sakura's latent influence seems to stir a sense of empathy, but Kama herself quickly ignores them as fast as they come.
Parvati focuses on Sakura's 'good aspects', sure, but she also takes the time to promise to Medusa that she'd take care of Sakura's body as long as she's using it. It'd be one thing if Parvati was especially callous towards Sakura in her own way, but that's not how it is. Parvati draws out Sakura's good, but also cares about Sakura. Kama draws out Sakura's bad, but is only doing it because that's what Parvati left and she wants to shame her without really knowing what she's shaming Parvati for. It makes Kama seem the most shallow out of the two.
Muramasa and Shirou have an unspoken connection because of who they are and what they're obsessed with, and so Shirou giving his body to a Servant to the sake of the Human Order follows. Ishtar and Rin have an unspoken agreement that allows Ishtar to use the majority of their shared body, as Rin commonly accepts they're one and the same. Parvati and Sakura seem to have a similar agreement, as Parvati is willing to let herself be openly influenced by Sakura's feelings and memories, yet desires to take care of the girl that lent herself to her. Astraea and Luvia, Zhuge Liang and Waver, Mannanan and Bazett- there's always a degree of input that the Servant defers to the host in order to properly manifest as a pseudo. Kama and Sakura also have parallels, but Kama is the least open to those parallels.
Kama doesn't understand humanity. She doesn't even really understand love in any sense. She barely understands the random teenager that summoned her. Which is what makes her a Beast, because she has a gross misunderstanding of human nature derived from a sense of misplaced love and compassion. Compared to Parvati, who acts with focused, if sometimes overly intense compassion towards those she cares about, be it Ashwatthama, Ganesha, or her Master, when it comes to which one of them 'understands love the most', Parvati is a much more competent 'god of love' than Kama, because Parvati understands that there's much more to 'love' than the surface aspects of it that Kama focuses far too much on.
While Parvati is the only one who is stated to pick multiple aspects from their hosts, that seems to fall in line with who she is as a goddess. Parvati herself is a goddess that exists as a 'benevolent form with darker manifestations', and so it only makes sense that Parvati would do the same. If anything, a 'Dark Sakura' Servant would be someone like Durga or Kali, a goddess that represents the darker, more violent sides of Parvati, yet aren't entirely evil either.










