If you can describe a dividing line for Western society which puts Japan on the outside and all of the Pope, Barack Obama, all members of Congress, Hawaii, Ireland, and France on the inside, you’d get a really good East Asian Studies paper out of it. If your definition of Western society excludes any group in that second set, it is clearly a useless definition.

(Ask a simple question, get a simple answer: yes. One has to be mendacious about definitions to come to any other result.)

I would say yes, post WW2. It's a wealthy capitalistic representative democracy which seems to be what people say when they mean "the West", at least in terms of economics and meritocracy.

I'm inclined to say yes as well, but it really depends on what aspect of its society we're referring to.

No. For two reasons: first talking of the "West Society" as a single entity doesn’t make sense. There are multiples societies such as the Fench, the German, the US ones which have common traits but also some differences. You could however put them under the concept of Western civilization.

Secondly, Japan is very different from the West. Not so much about the institutions which were calqued on Europeans ones after the war, but because of how people think. Having living there speaking a bit of the language, it is a totally different world with different values and most people are just fine living in their country and nor caring about the West or any foreign countries.