Child of Japanese residence status

I'm asking on behalf of a friend who is possibly interested in living in Japan for a period of time. He is 24 years old.

Nationality: European (does not need a visa to enter Japan as a tourist)

Mother: born in Japan to a Japanese mother. Logically she should have her own koseki and no, she was born a dual citizen, so she never lost Japanese citizenship even though she moved to Europe very young.

Koseki: mother passed away, but Japanese grandma is still alive. In theory, she should be able to know her full name in Japanese and her honseki. How would one go about retrieving the koseki touhon then? Are embassies any help? Does it need it be done from Japan?

What are therefore the best ways to try living in Japan for some time?

A) Apply for "child of Japanese" visa from Europe;

B) Apply for working holiday visa (it's also an option);

C) Enter Japan as a tourist, retrieve the koseki and apply for CoJ residence status.

I feel like C might be the "fastest"?