Another thing that no longer works is setting the power modes in xorg.conf. I just switched to battery again, and the graphics card is always in “performance level 2” (I have 0 - 3, see screenshot above). Even with
Option "RegistryDwords" "PowerMizerEnable=0x1; PerfLevelSrc=0x2222; PowerMizerDefault=0x3; PowerMizerDefaultAC=0x1"
which should set the performance level to 0. There is no difference between PowerMizerDefault=0x3 and 0x1, it is always at performance level 2.
So it seems I cannot control the card in any way anymore.
I can try to let the primary (fixed) battery run down to 0% and then do the 20 secs button press you proposed.