How to add a field to the Confirmation Email?

[DrByte]

admin/orders.php
added after line 124:
$html_msg['EMAIL_TICKET_1'] = str_replace('\n','',EMAIL_TICKET_1);

Order confirmations aren't handled via the admin side. So editing that file isn't going to accomplish your stated goals.
For the emails sent to the customer when they are placing their order, you'll need to edit the send_order_emails() method of the /includes/classes/orders.php class file.