Hi,
1. Instead of just {VAR:order.tracking_number} you could do it like that:
<a href="{VAR:order.tracking_number}">{VAR:order.tracking_number}</a>
2. You can't do that with tags. In that case, you would have to use pure PHP code like that:
<?php
if(!empty($data->cart->tracking_number)) echo 'Tracking number: <a href="'.$data->cart->tracking_number.'">'.$data->cart->tracking_number.'</a>';
?>
2. In the email you receive as a customer you have a link to it on the order number.
Go in the menu Customers>Email history and open the email sent to a customer for his order and you'll have the link there.
Note however that the link will only work if you're logged in with the customer account on the frontend.
Also, you can create a menu item of the type "order listing" or "user control panel" via the Joomla menu manager to provide a link for you and your customers to access your orders' history.