Private
None of us really knew what was going on until shit really hit the fan. Maybe we all could have done more. Me included. What's important is that Weiss needed you, and you were there. You went and got her and brought her home. And you're there for her now. That's really the best you can do.
As for the rest? I'm not sure if there's really a definitive answer. I wish there was, I really do. You've had this experience, as shitty as it was, so now you have a better idea of the signs so ifgods forbid you see them again, you'll have an idea what needs to be done.
Other then that? Just keep being the good person you are, Yang.