I'm guessing Nive kinda planned that to happen by stopping her blessing on the city or something? She even wanted to finish off the stepmother and sister in case they survived.
I'm sure Nive would've destroyed everything right after she heard the story, if Ninette had been fine with it, lol. I think I'm at least somewhat aligned with your guess. I highly doubt Nive was just pretending when she said that humans didn't understand the dangers that come with benefiting from the spirit's grace. Ninette's dad was contracted with a high-rank spirit, so presumably that'd be enough to hold off any disaster. But then when Ninette contracted with the area's high-rank spirit, and disappeared from the timeline for 8 years, the city no longer had the blessing, and thus... welp. I think where we differ is you're implying that Nive did it on purpose (highly plausible, though), whereas I think... unless Nive put her to sleep for 8 years, there was something else at play, and the town basically just happened to lose its protector? What I'm saying is, I don't know if it was fully malicious.
My real question is how did she end up being a 'sacrifice' again??? She's now contracted with the guardian of a city that's apparently considered special within the entire country. She hardly has a reason to go running back to her family... and she has no clue that the Duke Tilson is her Til, so...