need custom nav menu on certain pages on same url

Hello. I’m using Automattic’s “Powered By/Toolbox” theme. Have a child theme all set up, using both standard and top nav menus. I need to set up a handful of pages with a different version of the nav menu. I’ve searched and searched and tried a zillion methods, and if i can’t get the theme to do this, the site is pretty much wasted. The theme will let me create a third menu, but i can’t figure out how to place it…

Can’t do a multisite or dual install because i need pages to have the same name as my actual dreamweaver site…(no separate url, no subdomains or folders.)

There has to be a way to set up a template, or a category or something that will have a different main nav menu from the rest of the site, no? Doesn’t seem like it should be that complicado.

Please help! Am banging head on desk now. many, many thanks to all you helpful people out there!