Re: XC5 Login
You can't. I still don't understand the logic behind this but someone at XC decided that email is the only way anyone can use ever as username. Not only this but the email entered as login username is the only email customers can use for communication. In other words in XC5 username and email are one and the same and changing one changes the other...
If you want customers to have an option what to use you have to change tons of core functionality via custom module. And not only how they login but also to add in addition an email customers to provide so the cart can communicate with them.