You've sent the notice and some customers opted-in. As I understand it, the problem is with your vendor being able to facilitate the overdraft via ATM & debit card transactions; they cannot. Any overdraft generated by "plastic" would essentially be inadvertent on the part of your bank.
The problems seem to be 1) the potential loss of revenue and 2) the fact that some customers who want to overdraw their account will not be able to.
Your opt-in customers gave you permission to overdraw their accounts via plastic. You never promised that you would do so.
You may have a public relations problem, but you do not have a compliance problem. I suggest you just casually notify your opt-in customers that you have experienced some unexpected delays on the technology side and expect the system to be fully functional on XX date. (Your letter will be an advertisement under Regulation DD, include all of the extra junk.)
I would not discuss it with my regulator; this is a business decision, not a compliance decision and you should know a lot more about that than they do...
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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.