OK I have it working, but I'm not sure what I did that did it.
First, I attempted to see if it was a permissions issue. Since my instructions configured tomcat to run under a specific account, I gave that account (and group) blanket ownership of the files in the install directory (and below).This did not solve the problem. A coworker suggested replacing my tomcat-users.xml with the one in the distribution archive, which I did. I then added the manager-gui role to the tomcat user as follows:And at this point I can login as the user "tomcat". With it operational now, I'm a little timid about tearing it all back down and trying to reproduce my success. Hopefully this will help someone in the future, though.