1) Also A318/19/20, A300 &A310
2) Technically it is more than possible. If there's something in the SOP's I've got no idea.
3) Why can't the APU be switched on? If it's there and works it should be on. That also recovers all galley power.
4) No idea. I just fix them.
Any incorrections, please feel free to correct them.
Safe flying.