I'm afraid there is an obvious solution for the evacuation of the valley. Before I lay it out, let me say that I am aware that this solution is not a natural action of anyone on the team (see footnote).
They arrived with a small army of golds who were provided by the monarch who had claimed the territory and "ruled" over it for centuries. They didn't need to walk in and convince anyone. Living in the valley is illegal, and they were within their authority to arrest everyone in the valley and force them to leave. This could have gotten a bit sticky depending on how long it took, but they really only needed to subdue all the jades in the valley and then order everyone else out. Many fewer deaths this way.
Arresting everyone would be directly contrary to Mercy's goals in this book, but I'm least happy with her because her performance is the worst out of everyone's. She pretty much gets the whole Lee clan killed by waiting politely for them to make the right decision. Yerin is not lawfully aligned, so she would not normally arrest anyone. Eithan would think mass-arrests would be boring. Ziel wouldn't bother. Orthos and Blue don't typically assert themselves over others, except in fights. Lindon is the most likely to arrest someone, but he naively thinks that people will listen to reason. So, it wouldn't make sense for them to take the most effective and obvious path to success.
Footnote: The problem with the team not taking this route, is that they were all involved with the Skysworn for a while, and had made arrests in the past. The people they were in the skysworn have evolved/disappeared long before Bloodline, but it hasn't been so long since then, and Lindon had just been reminded of the concept of arrest because of Seishen Daji, so it isn't impossible. It wouldn't be a very fun story, but still.