Honestly, this doesn't seem to have anything to do with Community Auth. You mentioned that you have an existing users table; Where did that come from? The scope of the support I can provide is limited to Community Auth, and it's hard to help in a case like this because you've got other code that is part of the problem.
If you want to make a download available for me, and it has a full site backup of files and database, I'll install it on my local machine and let you know what's wrong. Keep in mind, due to my schedule this may take more than 1 day.
