Episode 10 framed it more between Youko and Nia than Nia and Simon. Likewise, Simon's pain at the end isn't because of anything Nia did, but because of his unresolved "aniki" issues. Nia is presenting an alternate perspective, but it's not creating a clash between her and Simon. And then you have the fact that I'd hesitate to call their relationship "love" at this point in time.

The proposal was more like culture clash. Once Nia understood the concept, she was perfectly willing to accept his proposal. It wasn't an issue that came about because of their love, but Nia's imperfect understanding of customs.