You can edit a USR account as often as you like but once you delete a USR account it can not be re added for security purposes. You will have to create a new USR account.
You must start Tomcat with the -Djava.awt.headless=true directive to avoid the error. This is also true if you installed Houdini into the Tomcat instance on OSX. Edit the Tomcat startup script named catalina.