As you require the creation of a username separate from the email address, which is 
different from most other websites, it follows that users will attempt to log in with 
their email address.

WordPress, which requires usernames, lets users log in via both email address and 
username.

This would be a easy fix ( where USERNAME = <blah> OR EMAIL_ADDRESS = <blah>), 
a usability improvement, and a reduction in support time/costs.