Option 1 is that you simply decline to open the account without any attempt to determine the legality of the customer's operations. You have that right. Precedents exist in the fact that same banks will not provide services to MSBs and cannabis dispensaries regardless of their ability to demonstrate compliance with the law. (My opinion is this is what most community financial institutions will do.)
Option 2 is that you would only open the account for an entity of state government involved in Internet gambling. (The largest promoters of gambling in the U.S. are state governments.)
Option 3 is that you would open an account for a customer involved in Internet gambling if the customer could document the legality of that involvement. Interestingly, the regulation lists the documentation requirements necessary to meet this burden. Accordingly, that list would represent your minimum requirements.
As a practical observation, the bank's choice of a particular option would be reflected in its policy and its due diligence procedures.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.