1. No. There are no exceptions if you do not complete the investigation. This is why it is "provisional". If your customer is already a thief, Reg. E will not help you.
2. The notice may be extremely brief, from the OSC "
The notice of error is effective even if it does not contain the consumer's account number, so long as the financial institution is able to identify the account in question." As Dan noted, you may hold back the provisional, but you must proceed with your investigation. Train staff to use a form and get the info needed up front. Having to go back to customers again and again for info, forms, police reports, etc. is not what Reg. E is about. It protects the customer, not you. (I didn't write it, don't blame me.)
If you cannot get any assistance from the customer, you complete your investigation based on what you do have. That may be very limited and may lead to a denial, but do your investigation. Done right, and closed, you don't have to reopen it for reassertion. Done wrong, you may hang yourself.
3.
Review this thread. FYI, John B and I are planning on a webinar dedicated to error investigation under "E" and V/MC rules. ACH type issues should be included as well.