Update -
Appears that password "user" allows login for mystery User account.
Firmware options for this fixed, built-in User login appear to be some subset of the Admin privileges but still seems to be places to make changes and/or stop services.
Password change for this account doesn't appear possible.
(I've *tried* repeatedly to call D-Link Technical Support for help with several issues and haven't had much luck - Either can't get through or placed on hold for 20+ minutes at which point I give up...)