With demo mode, I just mean that I start a game and let it run without playing it.
So… in most cases, the so-called “demo mode” starts automatically.
Btw: my problem has been resolved: the lockups were caused by the (standard) Pi 2 overclocking feature in the Raspi setup.
The dram shouldn’t be set at 500: that’s way too critical.
Rather change the value to 483 in the ~/boot/config.txt
Now, everything is ok, the issues are gone.