Does Japan even belong to sinosphere? It seems to me that despite sharing some superficial similarities by borrowing (read: copying) Chinese culture, they are a very weird people and have caused significant damage to China, either directly or indirectly, physical or spiritual (picrel).

I wouldn't say Japan has no responsibility to why the Chinese society is the way it is today. They are the first country in Asia to implant the mind virus of modernism and the idea that we NEED to "progress". Mao even thanked the Japanese for invading China because it indirectly helped the CCP to rise for power. Modern China is basically a completely new invention that is built upon the destruction of its old self.