Yes, a row in core.Logins merely indicates that user can login using the saved password (or, more accurately, a password whose hash matches the saved hash). If you remove the row the user won't be able to login using database authentication (LDAP will be their only option). The user's account is unaffected.

Site admins CAN change users' email addresses / user names. See the "Change Email" button on the details page for each user.

Adam