The Diamond Age question

Because that's how its known to the Japanese, and the man is a stickler for authenticity.

Because SO CYBERPUNK, BRO.

Serious answer: it's a shorthand for showing us that the setting is our world, but different. If it was actually because he was a stickler for authenticity, then China would be "Zhongguo," which he doesn't use.

Actually it is Nihon or Nippon in Japan. The Victorians are a major sect in this book. The British almost always refer to Japan as Nippon. So the usage of Nippon makes sense. Currently I think most Japanese will use Nihon more ie Nihon-gen, Nihon-go

Japan is the name of the main island, not the name of the country. the name of the country is the Empire of Nippon.