How to include most specific tpl files in a less specific tpl file?
It can be difficult to track the way different template files are used. First, I recommend turning on theme debugging by putting this in your settings.php

$conf['theme_debug'] = TRUE;


See this documentation for more information about theme debugging: https://www.drupal.org/node/223440 The main thing to know about that option is that it will print out the file names of the templates being suggested.

For your specific question, php includes are not the mechanism by which themes reference other themes. Rather the variables in one template are often "render arrays" (arrays with a very particular structure that sets up certain template files to be called). Here is the documentation on Render arrays in Drupal 7: https://www.drupal.org/node/930760 Ultimately, changing the template file selected involves working with render arrays in one form or another.