Cron error after upgrading to 4.7.14 - Function "is_admin()" is missing, even though WordPress is the user framework
The error message comes from here and indicates that cron.php is not bootstrapping the WordPress CMS, so WordPress functions aren't available.

WordPress is_admin() is defined in wp-includes/load.php. If WordPress was initialised, that function would be present.

If you are able to configure your cron to execute via commandline (eg wp-cli method or PHP CLI method) instead of by request to cron.php, that might give you a workaround for whatever issue is affecting this setup. (Edit: Oh, you tried that? Sorry!)

There was a previous report (from 2012) of the same behaviour: CRM-9727: cron.php will not run in Wordpress, is_admin() call to undefined function.