Guest reviews should be owned by the admin, as it has to be owned by a user in the Users table. I don't see why you would be getting the "Column Review Owner can not be left blank." error unless your admin user did not exist.
There was another bug in the code though, but it looks like you didn't run into it (the use of $USER when trying to send an email to the admin).
Adrian
There was another bug in the code though, but it looks like you didn't run into it (the use of $USER when trying to send an email to the admin).
Adrian