I think where you are getting confused is between server side parent paths and client side.
Web Wiz Forums, unless modified, does not use server side parent paths as these are disabled by default in IIS. If you use an unmodified version of web wiz forums it will work fine.
Parent paths in CSS or HTML are client side and can not be disabled by the web host as these run in the browser and not server side.
Only server side parent paths would be disabled, as already mentioned Web Wiz Forums does not use server side parent paths.
If you remove your modifications to call include files outside of the forums root using parent paths you will find it would run.