Congress could moot all of this by simply passing (or repealing existing law) a law that leaves the issues to the states in question. While online gambling poses issues when not all states adopt the same rules, just treat it like sales tax and require the vendor (and gambler) to obey the laws of the state of the gambler. Sure, there would be vendors and gamblers who don't obey this, but there are already gambling providers (bookies) and gamblers who aren't following state or federal law.