I found a fix. First I completely uninstalled Mathematica, including the folders in the /Library and ~/Library paths (Mathematica and Wolfram folders). Then I reinstalled Mathematica. This fixed some of the problems, specifically I no longer received the ExistsKeyQ error and the "demo" connection worked. However, the mySQL connector/J driver failed with a JDBC error. I downloaded the latest version from mysql.com and copied to the Mathematica.app path Contents/SystemFiles/Links/DatabaseLink/Java, where I removed the existing version. After that it worked.