Why do you believe Japan would let Taiwan stand alone?
Japan knows they will be next in the chain that the CCP hits:
Hong Kong
Taiwan
Japan
South Korea (maybe jointly via North Korea)
The Phillipines
Australia
India
The CCP will continue to push.
After Taiwan I see the Philippines next on the list, Japan a distant 3rd at best. If things ever get to that point Russia and China would become allies (of convenience) and Japan currently receives most of its oil from Russia, so Japan isn’t likely to be the one to start hostilities.