Load single template via plugin on custom url structure

function pss_load_templates( $original_template ) {

       if ( is_tax('plugin-procedure')) {
               if ( file_exists( get_stylesheet_directory(). 'taxonomy-plugin-procedure.php' ) ) {
                     return get_stylesheet_directory() . 'taxonomy-plugin-procedure.php';
               } else {
                       return plugin_dir_path(__FILE__) . 'taxonomy-plugin-procedure.php';
               }
       } elseif(is_single('plugin-gallery')) {
               if (  file_exists( get_stylesheet_directory(). 'single-plugin-gallery.php' ) ) {
                       return get_stylesheet_directory() . 'single-plugin-gallery.php';
               } else {
                       return plugin_dir_path(__FILE__) . 'single-plugin-gallery.php';
               }
       }
        return $original_template;
}
add_action( 'template_include', 'pss_load_templates' );

/** 
 * Tell WordPress how to interpret our project URL structure
 *
 * @param array $rules Existing rewrite rules
 * @return array
 */

function pss_add_rewrite_rules( $rules ) { 
  $new = array();
  $new['plugin-gallery/([^/]+)/page/?([0-9]{1,})/?$'] = 'index.php?plugin-procedure=$matches[1]&paged=$matches[2]'; 
  return array_merge( $new, $rules ); // Ensure our rules come first
}

add_filter( 'rewrite_rules_array', 'pss_add_rewrite_rules' );

/**
 * Handle the '%procedures%' URL placeholder
 *
 * @param str $link The link to the post
 * @param WP_Post object $post The post object
 * @return str
 */

function pss_filter_post_type_link( $link, $post ) {
  if ( $post->post_type == 'plugin-gallery' ) {
    if ( $cats = get_the_terms( $post->ID, 'plugin-procedure' ) ) {
      $link = str_replace( '%procedures%', current( $cats )->slug, $link );
    }
  }
  return $link;
}

add_filter( 'post_type_link', 'pss_filter_post_type_link', 10, 2 );


/* ====================
    Post Type: Gallery
==================== */

add_action('init', 'plugin_register_my_cpt_gallery');

function plugin_register_my_cpt_gallery() {

    $singular = __( 'Plugin Gallery' );
    $plural = __( 'Plugin Galleries' );

    //Used for the rewrite slug below.
    $singular_slug = str_replace( ' ', '-', $singular );

    //Setup all the labels to accurately reflect this post type.
    $labels = array(
        'name'                  => $singular,
        'singular_name'         => $singular,
        'add_new'               => 'Add New',
        'add_new_item'          => 'Add New ' . $singular,
        'edit'                  => 'Edit',
        'edit_item'             => 'Edit ' . $singular,
        'new_item'              => 'New ' . $singular,
        'view'                  => 'View ' . $singular,
        'view_item'             => 'View ' . $singular,
        'search_term'           => 'Search ' . $plural,
        'parent'                => 'Parent ' . $singular,
        'not_found'             => 'No ' . $plural .' found',
        'not_found_in_trash'    => 'No ' . $plural .' in Trash'
    );

        //Define all the arguments for this post type.
    $args = array(
        'labels'              => $labels,
        'description'         => '',
        'public'              => true,
        'publicly_queryable'  => true,
        'exclude_from_search' => false,
        'show_in_nav_menus'   => true,
        'show_ui'             => true,
        'show_in_menu'        => true,
        'show_in_admin_bar'   => true,
        'menu_position'       => 20,
        'menu_icon'           => 'dashicons-images-alt2',
        'can_export'          => true,
        'delete_with_user'    => false,
        'hierarchical'        => true,
        'has_archive'         => true,
        'query_var'           => true,
        'capability_type'     => 'post',
        'map_meta_cap'        => true,
        // 'capabilities' => array(),
        'rewrite'  => array( 
            'slug' => 'photo-gallery/%procedures%',
            'with_front' => false,
            'pages' => true,
            'feeds' => false,

        ),
        'supports' => array( 
            'title','editor','excerpt','custom-fields','revisions','thumbnail','author','page-attributes'
        )
    );

    //Create the post type using the above two varaiables.
    register_post_type( 'plugin-gallery', $args);
}


/* ====================
    Taxonomy: Procedures
==================== */

add_action('init', 'plugin_register_my_taxes_procedures');

function plugin_register_my_taxes_procedures() {

    $plural = __( 'Plugin Procedures' );
    $singular = __( 'Plugin Procedure' );

    //Used for the rewrite slug below.
    $singular_slug = str_replace( ' ', '-', $singular );


    $labels = array(
        'name'                       => $plural,
        'singular_name'              => $singular,
        'search_items'               => 'Search ' . $plural,
        'popular_items'              => 'Popular ' . $plural,
        'all_items'                  => 'All ' . $plural,
        'parent_item'                => null,
        'parent_item_colon'          => null,
        'edit_item'                  => 'Edit ' . $singular,
        'update_item'                => 'Update ' . $singular,
        'add_new_item'               => 'Add New ' . $singular,
        'new_item_name'              => 'New ' . $singular . ' Name',
        'separate_items_with_commas' => 'Separate ' . $plural . ' with commas',
        'add_or_remove_items'        => 'Add or remove ' . $plural,
        'choose_from_most_used'      => 'Choose from the most used ' . $plural,
        'not_found'                  => 'No ' . $plural . ' found.',
        'menu_name'                  => $plural,
    );

    $args = array(
        'hierarchical'          => true,
        'labels'                => $labels,
        'show_ui'               => true,
        'show_admin_column'     => true,
        'update_count_callback' => '_update_post_term_count',
        'query_var'             => true,
        'rewrite'               => array( 
            'slug' => 'photo-gallery',
            'with_front' => false,
        ),
    );

    register_taxonomy( strtolower( $singular_slug ), 'plugin-gallery', $args );

}