WordPress child theme fails to override parent navigation menu in /inc/structure/header.php

You need to copy to your child and edit the function where the file is included, probably ´shop_isle_primary_navigation´, to change the path where the file is located. WordPress only loads automatically from child the default files, that’s the ones defined in template hierarchy. You can read more about this at https://developer.wordpress.org/themes/basics/template-hierarchy/