Robin is, as far as I know, correct.
He did say he spent a long time at the department of transport trying to find a solution for you, after all.
If you want to buy a bike here, you need to be a resident. That usually means having a visa with a say of 6 months or longer. It is possible to "buy" from someone, but you will not be able to "register" it in your own name. In Japan, vehicle registration requires you to submit your residence document. You can only get the residence document if you are a registered resident. A tourist cannot register as a resident. If you had a student visa, for example, you could get residence documents.
Possibly, you could use a bike registered in someone else's name, but I suspect you would have troubles every time you cross a border. Maybe a document proving that you have permission to drive the vehicle would work, but I personally have no experience with this.
All other options would probably involve some less than legal solutions. I would not recommend trying to use forged documents in Japan, as that could get you in a bit of trouble (especially trying to get a Carnet using fake registrations documents, as that would possibly get you locked up for fraud).
Maybe the Gaijinriders forum for expats could help you. But I assume the answers will all be the same.

As for the rest of your questions,
Proof of a parking spot is not required for motorcycles, only cars.
The Hokkaido-Sakhalin ferry probably will be discontinued next year. There is still a ferry to Vladivostok.
CDP would require you to have the bike registered in your name and you would have to pay a large security deposit equal to more than the value of your bike. If you have that kind of money, you would save a lot of hassle just by buying in your home country and shipping to Japan. In any case, you would have to bring the bike back to Japan after the trip to get your money back.
This is the only CX500 I can find for sale in Japan: http://motorcycle.goobike.com/motorc...8B30110706004/
HONDA CX500 TURBO | 1982 | WHITE | 29,758

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Japan touring information