Issue with addition of existing user

An existing user can only be added if both the username and email are entered as it already exists. If a different username is added of the existing user, we get an error that username is not available and inbox needs to be checked for activation.

If an admin of a subsite of a network can not know whether some email id is already in use or not, s/he can not even know that what user id is used in case email id is already in use. So, it effectively defies the use case 3 described by you.

May be solution should be like this. If a user is already in use and site-admin enters different username than existing, the user should be added to his site with the previously used username and one message should be displayed telling the used username.

Thank!