The best may is allowed to be non-Orthodox. This is because Orthodox weddings do not actually include a best man. The tradition of best man is a Western one, so because it is not an Orthodox tradition, the best man is allowed to be non-Orthodox.
Of course. Your best man is the person you want to witness your marriage vows. It doesn't matter if your best man is married or not....or even a man for that matter. Your best friend should be the person standing by your side when you take your vows.
The best man is usually the Grooms best friend. He is equivalent to the maid of honor. This best man is also the person that holds the brides wedding ring and hands it to the groom to place on her wedding finger during the ceremony.
The best man is generally the groom's best friend or even a brother. There are no specific rules. If the wedding was taking place in another town or country and the groom did not have a best friend or brother present then he could have his bride's brother be best man or one of her male friends.