Under the beneficial ownership requirements, is the bank required to verify/confirm ownership percentage by obtaining documentation that supports what the customer is certifying (i.e. shares, etc.)?

Just what would you proposed to get to do that verification?

A headache

Although not required, asking for the percentage of ownership is a reasonable thing to do in connection with checking the OFAC list.

Attempting to "verify" what you are told is a fool's errand; there is no way to do it without relying on documentation which the legal entity customer's representatives could not easily fabricate. If they are going to lie to you, they will have no qualms about generating documentation in support of the lie.

Don't try to fix a regulation that was badly done...