There have been many well thought-out and well explained hypotheses in this amazing thread. However, I still think that the basic problem was one that could have been solved by Power and Attitude.
Decades ago I flew an aircraft that had a beaut, coloured Standby Horizon, powered by a 28v DC battery, which was in turn kept charged by the aircraft's main electrical system. In the event of a Total Electrics Failure, the aforesaid 28v battery would provide power for a guaranteed 30 minutes. It was completely independent of any other power source, and would get you out of trouble when all else had failed.
Why can't we have such a simple and effective solution today?