3
1) I don't think it's possible, since that field, like the other fields, is only used as a search criterion if you specify a non-empty value for it.
modules/system/admin/findusers/main.php
if ( !empty($_POST['user_name']) ) {
...
I prefer phpMyAdmin for that kind of process anyway.
2) The special tags available in email templates are handled in class/xoopsmailer.php, in function XoopsMailer::send(). X_NAME is not currently one of those tags.