Meta query with a sub query group possible?

I don’t think you need the relation parameter. It defaults to AND.

    $nowtime = time();
    $the_query = new WP_Query( array(
     'post_type' => 'promos', 
     'orderby' => 'menu_order', 
     'order' => 'ASC', 
     'meta_query' => array(
         array(
          'key'     => 'wpcf-type-of-promo',
          'value'   => 'walls',
          'compare' => 'NOT LIKE'
         ),
        array(
          'key'     => 'wpcf-date-and-time-start',
          'value'   => $nowtime,
          'compare' => '<',
          'type' => 'NUMERIC'
        ),
        array(
          'key'     => 'wpcf-date-and-time-end',
          'value'   => $nowtime,
          'compare' => '<',
          'type' => 'NUMERIC'
        )
     )
   ));