You need to add another node to this equation-North Korea. The US/Japan and South Korea have shown little inclination to even consider isolating the North, let alone go to war. Which pretty means that they would be even less willing to do anything about Taiwan. Would the South Koreans want to risk war with China over Taiwan knowing that the Chinese run the show in Pyongyang??
You are correct. It gets complex indeed when talking about interrelations within the Western Pacific rim.