If no post in parent category jump to first children

I have a wordpress blog, which i try to use as WebCMS, and i have certain parent categories with no content, but i would like to show the content of the next following children cetagroy instead.

Here is my current code:

<?php if (have_posts()) : ?>

		<?php while (have_posts()) : the_post(); ?>

		    <?php
          // check if the post belongs directly to the current category;
          // continue if not
          $current_categories = get_the_category();
          $found_category = false;
          foreach ($current_categories as $current_category) {
            if ($current_category->term_id == $cat) {
              $found_category = true;
              // counting level
              $level = 0;
              while ($current_category->parent != 0) {
                $current_category = get_category($current_category->parent);
                $level = $level + 1;
              }
              break;
            }
          }
          if (!$found_category) {
            continue;
          }
        ?>

Additionally i have two releated issues to that children content one.

1.) i use MyCategoryOrder Plugin, to sort the categories and i wuld like for the display of the content, to keep the same order/structure:

<?php wp_list_categories('orderby=order&show_count=0&title_li=&depth=3'); ?>

2.) by clicking on the parent category, my navigation menu starts displaying the children. Can i somehow mark the first children (which content should be displayed) with a “current_cat” css-class? So that the users know where he/she is?

Thank You for your support in advance,
bb