getting parent page id when using custom menu.

Not sure if this is robust enough, but it shows basic traversal through menu to look for current post first then for its menu parent:

/**
 * @param mixed $menu
 * @param int   $post_id
 *
 * @return WP_Post|bool
 */
function get_menu_parent( $menu, $post_id = null ) {

    $post_id        = $post_id ? : get_the_ID();
    $menu_items     = wp_get_nav_menu_items( $menu );
    $parent_item_id = wp_filter_object_list( $menu_items, array( 'object_id' => $post_id ), 'and', 'menu_item_parent' );

    if ( ! empty( $parent_item_id ) ) {
        $parent_item_id = array_shift( $parent_item_id );
        $parent_post_id = wp_filter_object_list( $menu_items, array( 'ID' => $parent_item_id ), 'and', 'object_id' );

        if ( ! empty( $parent_post_id ) ) {
            $parent_post_id = array_shift( $parent_post_id );

            return get_post( $parent_post_id );
        }
    }

    return false;
}