Turbine engines with FADEC have a Permanent Magnet Generator (PMG) driven by the engine as a back-up to the A/C bus, so that if all electrics fail the engine looks after itself.

How did these light aircraft engines get certified so that a single failure can cause disaster?

Piston engines have 2 magnetos for back-up.

Two batteries + a small one (DA42 if I remember correctly)

Actually fitting of the ECU backup batteries in the DA42 was mandatory, EASA AD 2007-0183R2, same batteries as altenator backup, only it is two in series for each engine.