Their film and movie titles are coded like that so that a consumer is able to find them almost anywhere. A product code on R18.com is the same as it is on Zenra, same as it is on AllJapanesePass, same as it is on JAV Giga, same as it is on JAVHD etc. Same as it was on Erito (before the Manwin/Mindgeek acquisition) I've been to a few Japanese websites whose content being featured could be highly objectionable and controversial but the product codes are still the same for the unrelated film that I was looking for.
Most Japanese AV Idols are famous in their own way. You mentioned Maria Ozawa and Reiko Kobayawa. Yes, they are well known in Japan and in the West. However, Hitomi Tanaka is probably the most notable Japanese AV Idol currently known all around the world. But there are plenty of others, Yuka Haneda, Ruri Saijo, Anri Okita, Yuki Touma, Meguri Fujiura, Rin Aoki, Riku Minato, Rara Anzai, Ichika Kasagi, Yume Kana and Tomoko Kamisaka are all famous here and as well in Japan.
Japanese women don't start out as Japanese AV Idols. Every lady starts out as a Gravure Idol. The difference between Japanese Gravure and AV Idols is that gravure idols tend to be ladies who are models and singers and entertainers. While AV Idols tend to also be models and then they get into nude modeling and Japanese porn. AV Idol means "adult video" Idol.
Three famous Japanese Gravure models are Yoko Matsugane, Yuzuki Aikawa and Yamada Nama.
Mainstream how? Culturally in Japan though it's more restricted than anything else because everything -- both male and female genitalia -- are all pixelated out. You also won't see many Japanese pornstars (both women or men) with tattoos. And if you do see tattoos occasionally, they will be on the American, Chinese and Canadian female pornstars who live and travel to Japan. A male Japanese pornstar may have an occasional tattoo here and there. But those tattoos are much different than what the Yakuza have or certain members of the underground Japanese organized crime clans or families have. Soldiers in the Yakuza have tattoos all over their bodies but nothing that can be seen. Nothing from the collar and neck up and nothing from the wrist out to the fingers.