Dynasty Warriors: Origins (NG+)

So my first play-through I went with Wei and didn’t know about the true ending at the time, how would it work if I wanted to unlock the new ending? Would I have to do the whole Wei campaign again whilst getting the requirements for the alternate ending? Or can I just do said missions by selecting from story mode at the inn?

I saved Dian Wei and Guo Jia in separate missions from the story menu and got the ‘fate altered’ messages but when I start The battle of Chibi I cannot select Guo Jia as a companion.