Use wp_nav_menu to display ALL pages

You could move away from wp_nav_menu() completely and instead use wp_list_pages().

See the codex for more information, but I believe this may be what you are looking for.

http://codex.wordpress.org/Function_Reference/wp_list_pages

I do understand that this function outputs the pages. The point I was trying to make though is that you can use this in conjunction with CSS to get it looking like a menu. I made an example page that includes this code:

    <?php
/*
* Template Name: Testing
*/
    ?>
    <link rel="stylesheet" type="text/css" href="http://nickyoungweb.com/zip/wp-content/themes/THS2012/style.css" />
    <nav id="nav">
         <?php
            wp_list_pages( array(
                'title_li' => ''
            ));
         ?>
       <br style="clear: both; ">
    </nav>

I have it linked to a stylesheet that contains this code:

    /**** WP LIST PAGES NAV *****/
    nav {  }
    nav li { list-style-type: none; float: left; }
    nav li a { width: 50px; height: 30px; background: #000; color: #fff; padding: 10px; margin: 2px; }
    nav li a:hover { background: #f00; color: #000; }
    nav ul li { display: none; }

It is a very basic example of how you could make it work with this function.

Hopefully this helps clear up any confusion I might have caused you and also gives you an idea of how it CAN be done with this function.