Aha. Found it.
For future reference, the plugin Genesis Layout Extras has a setting which allows you to override the Default layout and force a content/sidebar EVEN if the sidebar is Unregistered in the child theme.
Options are to either deactivate the plugin or reset to the default genesis layout for the home page, which will then render without the primary sidebar for this theme.
Only thing left unanswered is COULD I leave that Genesis Primary sidebar on the home page, if I were to increase the theme width, and still have it work in a responsive environment?
Thanks.