I think you have to give the user the ability to create articles / pages and save the customization settings to really get an idea of the theme.
I would create a new admin user role that has all the permissions of an administrator, with the exception of creating / deleting users. This way, you can not stay out of your site. Then I would create a function by hanging the
wp_logout action hook to delete all items / pages, and reset the customizer settings to the default values when the user logs out.