I installed MySQL from the official package, and I broke the back-end data, and now no-one can log in, and no one has privileges, to connect.

Anyway, how can I COMPLETELY remove everything MySQL put on my computer, so I can cleanly re-install it, and revert to a backup I have.

You can use DesInstaller to remove everything that the .pkg file installed originally, or you can 'sudo rm -rf /usr/local/mysql-VERSION' in the terminal, replacing VERSION with the version that you installed. You can run 'ls /usr/local' to see the exact file name, mine is "mysql-standard-4.0.24-apple-darwin7.7.0-powerpc".