UPDATE 2/7/2018...
I uninstalled 757 III (+ Hotfix 2) and reinstalled the base only. Loaded P3D with the aircraft and the aircraft came to life (I have a switch selection for Avionics Master On/Off). I took it for a spin around the airport (with some obvious autopilot issues) and landed. Closed P3D and loaded Hotfix 2. Then I had the powerless aircraft again.

There's something with Hotfix 2 causing the problem for me.  >:(