Okay, so there may be a better way to do this, but what I seemed to find was the following based on some of the discussion.
Want to be able to navigate from category to category and see the "sub-categories" in the center of the page, but... When reaching a category that has product (which in ZC in default coding is the way things are expected to be either a category has active product or other sub-categories but not both active product and sub-categories), then display product "0" for that category which is likely to be based on some sort of sort operation...
I didn't chase down every possibility, and there are "cleaner" ways to do this than to literally change code like to use a notifier/observer operation, but during testing a little code mod first could make the difference/easier. :)
Anyways, so that full process is controlled by the includes/modules/pages/index/ code in that folder basically..
When one navigates to/through the categories or back to the main (default) screen, then the code in this area is evaluating the conditions to see what page (listing) should be displayed...
In your case, when a visitor tries to arrive at this "last page", you would like them diverted to the product that would have appeared first on that page... So... here's my thought, which does involve modifying a "non-overrideable" file...
If you open includes/modules/pages/index/main_template_vars.php and then at/around line 196: $tpl_page_body = 'tpl_index_product_list.php';
And just before or just after this, if you add this line which comes from includes/modules/pages/index/header_php.php:
Code:
zen_redirect(zen_href_link(zen_get_info_page($listing->fields['products_id']), ($cPath ? 'cPath=' . $cPath . '&' : '') . 'products_id=' . $listing->fields['products_id']));
Then this is likely to at least *prove* the theory. The next thing would be to ensure that the particular products_id is provided/available if in fact some sort order was applied, but this should redirect to the first item of the return list.
Please try this on your local development site.
As to my "setup". If you mean how am I navigating to see things currently, well, good ole' blackberry... :)