1) going to just/spam box depend on so many things. You can move some emails from junk to inbox and check if that helps.
2) It looks like your file have permission issue. Did you changed file permission? If yes and still have same issue You can move fep-email-piping.php file directly to wp-content directly and changed this file permission to 755. then point to this file path in email forwarder setup
Let me know.