... however, if the Bank's disclosures state thus, then the Bank can require a written (read WSUPP or otherwise) statement [205.11.b.2.] I've always understood this aspect.
While 205.11(b)(2) allows the bank to require that an error claim be reduced to writing, it does not permit the bank to condition ANYTHING except provisional credit upon receipt of that writing. The oral claim remains valid, and the bank has to comply with everything in 205.11 except the provisional credit requirement.
As for supplying a copy of the WSUPP if requested by the ODFI, the NACHA rules allow the WSUPP to be faxed or sent via other electronic means (by the customer). Presumably, an email from the consumer or a WSUPP created in your online system is authenticated in that the consumer initiated the message from a password-potected environment. I'd question a fax of an unsigned WSUPP form. If I were an ODFI and had a large sum on the line, I'd think about challenging it.