Hello bergyver,
Here's my answer. If you are unable to log in to one of your added user. You can try the Forgot Password option for that user and enter the email address for that user, it will send you a link on users email address to create new password.
Secondly , there is a difference between the Method Id email address and email address mentioned under User section. The email address mentioned when you click on users is the email address enables you to send email through method. Therefore, if you make any changes to the email address it will only affect your email preference settings.
-Inder