Add multiple orderby with pre get posts

You could try using the posts_orderby filter:

add_filter('pre_get_posts', 'sort_arc');
function sort_arc($q) {
 if ($q->is_category() && $q->is_main_query()) {
    $q->set('meta_key', 'the-year');
    add_filter('posts_orderby', 'custom_posts_orderby');
 }
 return $q;
}

where

function custom_posts_orderby($orderby) {
    global $wpdb;
    return $wpdb->postmeta.".meta_value+0 DESC, ".$wpdb->posts .".post_title ASC";
}

takes care of the custom ordering. We use meta_value+0 from the meta_value_num definition in /wp-includes/query.php.

Update:

Here is one idea how you could try to combine the two hooks:

add_filter('pre_get_posts', 'custom_pre_get_posts');
function custom_pre_get_posts($q) {
    if( $q->is_admin ) {
        return $q;
    }
    if( !$q->is_main_query() ) {
        //echo "DEBUG: !is_main_query() <pre>".print_r($q,true)."</pre>";
        return $q;
    }
    if( !$q->is_archive() ) {
        //echo "DEBUG: !is_archive() <pre>".print_r($q,true)."</pre>";
        return $q;
    }
    if ($q->is_category() && !isset($_GET['sort']) ) {
        $q->set('meta_key', 'the-year');
        add_filter('posts_orderby', 'custom_posts_orderby');
        //echo "DEBUG: is_category() <pre>".print_r($q,true)."</pre>";
        return $q;
    }
    $custom_field = ( $_GET['sort'] ) ? stripslashes( $_GET['sort'] ) : '';
    $custom_value = ( $_GET['sortorder'] ) ? stripslashes( $_GET['sortorder'] ) : '';
    if( $custom_field ) {
        $q->set( 'meta_key', $custom_field );
        $q->set( 'orderby', $custom_field );
        if( $custom_value ) {
            $q->set( 'order', $custom_value );
            //echo "DEBUG: custom_value <pre>".print_r($q,true)."</pre>";
            return $q;
        }
        //echo "DEBUG: custom_field <pre>".print_r($q,true)."</pre>";
        return $q;
    }
    //echo "DEBUG: else <pre>".print_r($q,true)."</pre>";
    return $q;
}