It's true that he never said he wanted her to be subservient himself. But he said he doesn't care that this marriage nullifies everything she's worked towards as long as he gets what he wants.
It was nothing but an overdramatisation, quite fitting for the youth, though. The truth is, Latos isn't really working for anything. She took over the brother's indentity as an emergency measure, mostly to fight against the collapse of the family. But it's not really a solution for anything, it was just postponing finding a real solution. Apparently it would have been unthinkable for the family to have a female heir, especially one lacking fame. However, how long could this theater last anyway? Miraculously it has lasted this long, but it's not like Latos could marry a woman and save the family line. She also doesn't have a gender identity disorder; she doesn't want to be male. Sooner or later this had to happen. Due to Latos taking over his brother's identity, it could be postponed this much.
On the other hand, seeing how Latos still is around, pretending to be her dead brother, it means she is still sticking to the emergency plan. If she didn't care about her family, she would have hit the road a long time ago, as a woman most likely. Seeing how she wants to save the family, then a marriage such as this was inevitable.
And that means it will end with her in a subservient, passive aristocrat's wife's role because that's what her father expects and demands and his will is going to continue to be important because he's behind all of this and adopting her fiance into their house. She wasn't allowed to practice magic when her brother was alive. She only got to do it because she got him to allow her to attend school masquerading as her brother. The reason she's so afraid and depressed about the betrothal is because it means the end of everything she's worked for. And there's every expectation that he would know that after he and his father met with hers.
That's something she would need to discuss with Theria. Like I said before, since Latos is still around, she means to save the family, so a marriage was an inevitability sooner or later. Nothing says Theria would be against Latos continuing to practice magic. Their best bet would be to silence the old man together. That's what would need to happen no matter whom Latos marries. If she never planned to marry anyone, the family would die out, so why bother to pretend to be her brother this long?
The fact that he recognizes her hard work and still chooses a path that disrespects it does say a lot about him: It says that at his core all he cares about is getting what he wants.
there's no positive way to spin "even if I have to stomp all over her effort... I want this." It's very, explicitly, blatantly a declaration that for him the ends justify the means even if those means are destroying everything Latos has worked for. It's him admitting that he will take her as his wife regardless of the consequences to her because what matters to him in the end is his desires. This is his salvation, not theirs.
And how would you save the situation? The family line will only continue is Latos gives brith to children. Is your solution that Latos never marries anyone, but does reveal her true identity later once she's powerful enough? Then she just gets impregnated by random guys to produce possible heirs. Is that it? It wouldn't trample on her leadership role and wouldn't allow the father to push her into a subservient position in favour of a husband. Because there wouldn't be a husband, only fatherless children.