Category Template Hierarchy 'descendant-of-category' templates, allow any descendant to inherit the template of a set of parent categories.

category-template-hierarchy-descendants.php

add_filter('cth_category_template', 'descendant_templates');

/**
 * My plugin will call your function, and pass it the list of all templates
 * before sending them off to find which ones match.
 */
function descendant_templates($templates){
    
    if(is_category()){
        
        /**
         * The $parents variable is key, this is what I am not able to know
         * from within the plugin.
         * 
         * Im assuming you may have more than one top-level like collections.
         * The order given here will determine the hierarchical order, should
         * A particular category be a descendant of more than one of the $parents.
         */
        $parents = array ('collections', 'other-top-level-slug');
        
        //This will be populated with template names of matched categories
        $desc_templates = array(
            'slugs' => array(), 
            'ids' => array()
            );
        
        foreach($parents as $parent_slug){
            
            //This function is in the Template Category Hierarchy plugin.
            if(is_child_of_category($parent_slug, null, false)){
                
                $parent = get_category_by_slug($parent_slug);
                
                //Collecting slugs and ID's seperately so we can insert them in the right places later
                $desc_templates['slugs'][] = "descendant-of-category-{$parent->slug}.php";
                $desc_templates['ids'][] = "descendant-of-category-{$parent->id}.php";
            }
        }
        /**
         * Adding the appropriate elements to the templates array.
         * The position '2' for slugs can be assumbed because if
         * the current category is a descendant of any category, then
         * it will already have the is-child-*.php templates in the
         * list.
         * 
         * @uses array_slice();
         * @link http://php.net/manual/en/function.array-splice.php
         */
        array_splice($templates, 0, 0, $desc_templates['slugs']);
        array_splice($templates, 2, 0, $desc_templates['ids']);
    }
    
    return $templates;
  
}