The media reports the engine "spluttered" and the aircraft turned back, with the usual result. Conspiracy theories are ludicrous.
If the aircraft was the one I think it was, I am surprised the ATSB issued (according to the media) an immediate interim report, indicating a Log Book and history "clean bill of health."
I think you will find the FDR currently being discussed for light aircraft is a small and relatively low cost solid state device, currently under development and trial.