Live and work in Okinawa!
 

im from Russia but currently in US (im not a US citizen!)
My gf is from Okinawa.
I want to live and work in Okinawa.
I never been to Japan.
What are the possible ways to get a visa besides "as Visitor" for 90days?
Business visa?
How to find a job there and get permission from company IF im currently in US?
What r the possible ways to stay in Okinawa by getting at least Green card?

I think your girlfriend would be the best person to ask.

Can i get married to a Japanese National during "90days as a visitor" and what is going to happened then?

of course you can get married to a japanese national of opposite sex during your 90 days. at the time when you hand in the marriage application, nothing will happen. perhaps the people at the city hall will congratulate you but i can't think of anything else. perhaps you want you picture taken or something.

surely Japan will need to know if the marriage is genuine and not just an easy way to stay in a county

It's not as easy as simply getting married. First of all, you may need to show some kind of proof that your relationship has been going on for some time. Usually letters with dated postmarks are used for this. Also, most people get married after more than one visit, so travel tickets for previous visits could be used. You'll also need to provide an affidavit from the government of your country proving that you are not already married to someone else in your country, and you'll have to get this affidavit translated into Japanese.

But wait, that's not all. Before you'll be granted a spouse visa, your girlfriend will have to show that she earns enough money to support the both of you. If she is a student, or has only a part-time job, this will not be possible. In this case, you may have to get her family to sponsor you, provided that they make enough money to support both you and her.

The only other option is to like others do and get a proper work visa. For this you'll need a 4 year or better university degree, and sponsorship from a company who is willing to hire you.

The only thing you need to get married is the last - the proof of ability to get married.
Proof of relationship, etc, is what you need for a spouse visa. You can get married for whatever reason you like, even having only met the other person for 5 minutes to fill out the form. They don`t care. They only do care when you are trying to get permission to stay in the country on the basis of the relationship.

Same deal really, but you can get married even if there is no relationship. Just forget about getting a visa out of the deal.