Well we are by definition talking about an upheaval of the existing social order. And the "ruler must be a queen" thing is probably more fragile than you're giving it credit for. We've heard a lot how important it is for her to uphold an image of purity, what happens if she fails at that? A crisis of faith in the population, that's what. That moment is ripe for significant changes. Once the illusion is broken, do you think they would just replace her with another pure, perfect queen? "Sure, the last one turned out to secretly be a huge slut, but this queen is the real deal!" Personally I think that once the people see she's really like, that would not just break their faith in her, but also in the idea of having a perfect, pure queen. I don't think it's that far-fetched that they would fall back on something familiar, and Zetan culture is male-dominated and values strength. Them having a queen instead of a king is an anomaly in the first place.
Given that, I don't think that it's impossible that they would accept an emperor conquering their queen. Does that mean I'm actually predicting that's where the story is going to go though? Hell no. I don't think it would quite fit with the tone of the story up until this point, although he has kind of done that exact thing to the raider faction now that I think about it. Is conquering the other factions and becoming leader of them where things are going after all?