the action of the form is to post to the contact-us-confirm.php:
<form action="contact-us-confirm.php" method="post" name="form2" id="form2">
When working with forms, remember this rule:
All form Processing must occur on the action page
this means that the validation needs to be added to the contact-us-confirm.php page.
this is similar to the way the eCart checkout form works, see this thread bout adding validation to the confirm page of eCart for details on how to work with a form that post to another page: