I guess, there's a little bug with user questions list:

else if($this->action == 'user_questions') {
$user_questions = &$model->getUserQuestions();
$categories = &$model->getCategories($catid);
$this->action = 'open_questions';

Shouldn't it be $this->action = 'user_questions'? Because right now it doesn't return anything if no questions found, but it should according to this:

echo JText::_("NO_RESULTS");

In short, NO_RESULTS string is not there, because user_questions template never fired.