iirc I was worried about the CB because I had read something similar but I made sure to select "War for the Emperor" which was my only option, after I annexed Kyoto an event fired and I became shogun (though I think I was still Takeda at this point). Then I took the last daimyo province (Hosokawa), Hokkaido, and then used the "Japan is United" decision which made me Japan, changed my government type and gave me some MP. I stuck around for a while because I read somewhere that I might need Okinawa but that didn't cause it to trigger either.