I think that off-chain sportsbook can also meet some parts of on-chain by implementing something like say auto send payout of per bet on win to an address set in profile setting but still you need account but maybe something like sportsbook accepting only bitcoin address as username would be cool. Just my thoughts 
Not fealible. Risk control, a big problem.Unless they at the same time lay the player's stake to bettingexchange or other's bookie.
If so, they need to reduce the odds to ensure profits, like directbeteu.