some unsolicited thoughts about why Rashta is not Trashta:
Rashta is using the only methods she has available to her in a patriarchal and feudal society even more fucked up than our own to claw her way out of a shitty and abusive situation— and she's doing it with great skill tbqh. Even though she's the antagonist to Navier I literally cannot blame her for anything she's done so far.
Navier and Rashta are both in situations where no matter what they do they cannot truly win. If Navier asserts her dignity as the empress, it makes her look like a petty bully, and makes Sovieshu hate her even more, which undermines her social standing and authority. If Navier stands back & lets Sovieshu parade Rashta around high society, it undermines her dignity and power as the one and only empress, and (again) she loses social standing and authority. The only way for her to win is to exit the game— which eventually we know she does, by taking advantage of Sovieshu's decision to divorce her (she can't even divorce him on her OWN initiative, she has to wait for him to pull the trigger!) and attaching herself to yet another powerful man in a competing empire. Of course we know that Heinley worships the ground she walks on and will treat her right, but still, this sucks.
For Rashta it's even worse: if she stays in Navier's shadow as Sovieshu's mistress, her position is only tenable as long as Sovieshu is in love with her and doesn't get bored of her and discard her, plus she's constantly under threat from Navier & her sympathizers (we readers know Navier probably wouldn't outright try to have Rashta murdered, but Rashta doesn't know that). If Rashta tries to leverage her pregnancy and climb the ladder of legitimacy and gather supporters of her own and push to be made empress herself, she becomes MORE of a threat to Navier and the aristocratic faction and MORE likely to be murdered in the short term— but if she succeeds, she'll be much less vulnerable to being thrown out simply because Sovieshu lost sexual interest in her. And if she gets tossed out it doesn't just mean no more fancy dresses and jewels and party time with the emperor. It almost certainly means a return to slavery and sexual abuse and coercive pregnancy, forever.
Of course we know Sovieshu likes 'em young and dumb, so even if— BIG "if" at this point in time in the story— Rashta succeeds in becoming empress, she's just setting herself up to be in Navier's shoes next time Sovieshu finds some sadder, prettier, younger girl to lavish his attention on, and the cycle will repeat, and probably Rashta won't miraculously find a much better prince from some other empire to marry instead, but even if she does, the end result is that neither of these women are actually ever free to make decisions about how to lead their own lives independently.
Sovieshu is a trash misogynist even by the standards of the world in the story, I hope he dies lonely, the end, thank you for coming to my TED talk