Mac: Cannot run demo, even after downgrading to java 1.6

Environment: Because Oracle Java 1.7 makes the Minecraft demo crash (MC-998 et al) I downgraded to the Apple 1.6 browser plugin following the instructions on http://support.apple.com/kb/HT5559?viewlocale=en_US

Symptoms: The demo opens a security warning at startup, where one can only "deny" Minecraft access to the computer (even after checking "always trust"). See screenshot. Doing so will abort the demo.

Reason: The certificate is expired since Aug 2009. See screenshot.