Couple of things to think about:

-Sakhalin ferry is a nope. A little research and you'll find plenty of posts that confirm that. It is a passenger ferry and does not take vehicles.

- there ARE ferries that service Vladivostok (via sKorea) to Japan. The principal one is DBS Ferries, and it is now closed defunct since 2019. Ive been talking to Tatiana who is still answering her professional email, and there is a slim chance it will reopen in the future.

-travelling and arranging the ferry is normally easy. See https://www.madornomad.com/how-to-ge...ssia-to-japan/

-the CdP is not necessary and is expensive. Not really worth it for Japan>Europe travel.

-Meanwhile the CONIV19 has closed the Japanese border indefinitely as per the MOFA website. https://www.mofa.go.jp/ca/fna/page4e_001053.html

-insurance taking a bike FROM Japan to Europe will be complicated. Getting insurance in Russia is easy, many gas stations will do it, and it is not expensive. However i understand that getting insurance for Europe (green card) is quite difficult. Seems to me that there is a moto company out of Bulgaria (i think) that can do it and has good reviews. The name momentarily escapes me, but you can easily search HU for the answer.

-i suspect to convert a Japanese plated moto to European plate will be horribly complicated if not impossible. Here in France i would not even try.

-passing borders (turkey) is no problem having a passport different than the bike. As long as the bike is registered in your name (or you have a notarized or similarly official authority by the owner to use the motorcycle) you can cross having completed all the TIP paperwork. Source: Canadian using French plated moto.

I left my EU bike in Japan in the winter of 2019. Looks like I won't be getting near it for the next year or so...
Good luck.

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2023 back in Japan on the road after COVID, 2019 Paris Siberia Japan tbd -2010 WR250R; 2015-2016 Paris-Mongolia and Paris-Silk Road; 2011-2013 Canada Ushuaia