Get you a friend like Eiji who cares enough to smack some sense into you.
I'm glad it seems this isn't going to be dragged out in any manner, if we're jumping to Souta confronting her & her parents in less than three chapters from the point Nagi said goodbye to him. The pacing makes me feel far less conflicted about introducing this particular plot point.
And it sorta makes sense - it's an easy way to install drama into the narrative, while addressing Nagi being adopted, and also addressing her history of not speaking her mind or really having any sense of self-possession. Souta cracked her small world open, and now he's going to help her find her agency and ego when it comes to the direction of her life.
Yes, there might have been "better" ways to do so than invoke an arranged marriage plot device born out of a misunderstanding by her parents, but as-is, this is at least on-theme for a lot of this sort of story with a "well-bred ojou-sama" type heroine character, so it'd be familiar to long-time readers of the genre. Thus, it's a "safe" bet, and since it's not being dragged out and filled with multiple chapters of melodrama (at least it seems at this point), I think the author can be granted leniency.
I'm sure that given time, I could think of a different way to address Nagi's need to be more assertive and be okay with having wants and desires, but I think I've talked myself into being okay with how the narrative's progressing to hit those points, now.