How can I have the nodes grouped in descending order and the nodes in ascending order?
You can use hook_views_query_alter



function MODULENAME_views_query_alter(&$view, &$query) {
   //print_r($view->query->orderby);
  if ($view->name == 'VIEWNAME') {
  // check the position on the orderBy function in the query
  // For this you can check the values with print_r() function.
   $view->query->orderby[0]['direction'] = "ASC";
  }
}