the email address "username" is not valid.

When checking out if a user has already registered an email address if you were to revisit the site and type your email address and leave the radio button as No then you see an error message:

The e-mail address "username" is not valid.

Would it be better to say that the user email address is already registered, have you forgotten your password?

My setup is:

Continue without register: YES
Use username as order email: YES
Use anonymous checkout as option to login form: YES

I've also applied the patch from #1 on https://www.drupal.org/node/2392553 which works great for me!

With my setup I've also removed the register form from the checkout redirect process as there was another bug where if a user was going through the checkout and decided to create an account the login radio buttons were shown on the register page and the default was set to No, which caused an issue with registering, I wrongly raised an issue in the email_registration module here:

https://www.drupal.org/node/2535688

Thanks
Max