How to call wp_list_comments() with callback outside the loop

Just go ahead and call it in the same way the tventyeleven does:

<?php
                /* Loop through and list the comments. Tell wp_list_comments()
                 * to use twentyeleven_comment() to format the comments.
                 * If you want to overload this in a child theme then you can
                 * define twentyeleven_comment() and that will be used instead.
                 * See twentyeleven_comment() in twentyeleven/functions.php for more.
                 */
                wp_list_comments( array( 'callback' => 'twentyeleven_comment' ) );
            ?>

wp_list_comments function will manage $depth and other $args itself (in walker). See source code of wp_list_comments and the walker using callback arg