Actually there is a display name already, so the user name could just be
disabled. The main argument for a user name is to use it for technical
identification (e.g. or where a full email address or a display
name with special characters or white space wouldn't work so well.

Of course, at the very least, there is no reason why it shouldn't be
possible to log in using the email address instead of the user name
(it's unique, too). I'll add that as a first step.

Well, You'll need to relax your username requirements if you do that...

My email is
0b1100110 is rejected on the basis of: "Usernames cannot begin with a number."