Hello Donncha,
sorry about my delayed reply here.
I tested your latest fix. As soon as the WP Super Cache WP Touch plugin is turned on and you visit a page on mobile, the main cache file for the page gets the mobile version. So visiting it again on PC shows the mobile site.
I only did a quick check to see if I can figure this out and I:
1. Figured out it's not any of these action
add_cacheaction( 'wp_super_cache_mobile_browsers', 'wp_super_cache_wptouch_browsers' );
add_cacheaction( 'wp_super_cache_mobile_prefixes', 'wp_super_cache_wptouch_prefixes' );
add_cacheaction( 'wp_cache_check_mobile', 'wp_super_cache_wptouch_cookie_check' );
2. Figured out that .htaccess has no effect on mobile caching if you are using mod_rewrite caching. It seems that all of it is going through PHP. Please correct me if I'm wrong.
Thanks,
Martin