RESOLVED:
An Apple TSC and a member of the CE team got personally involved and scrutinized everything. Seems the problem was with a false computer object with the same macAddress that was causing the test client to fail. This was either causing the other clients to fail as well or they were false failures. Lesson 1: Clean up your computer objects. Lesson 2: It may take multiple restarts/shutdowns for the MCX Preferences to apply to a computer client.