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Sorry, which thread is this following up on?
If we discussed this I don't remember.
Where does your cpt/maintaxonomy/listing-name URL structure come from?
Taxonomies don't appear in URLs for posts (which is what I assume you mean by listing-name). With standard posts you can add %category% in the permalink structure, but not with custom post types and taxonomies, for which you need a custom rewrite solution.
So, before going any further in looking into how to make this work with sub-categories, can you describe your current solution?