To reply to your second question, you are correct that if the APU were running, then the electric motor pumps would pressurize the left and right hydraulic systems if both engines failed. However, if the APU were running, it would mean fuel was on the airplane and the engines had stopped for some other reason, such as dual mechanical failure. Dual mechanical failure is an extremely rare event. Running out of fuel is a much more likely reason for dual engine failure (along with volcanic ash or massive rain ingestion). No fuel means no APU, so you can't count on it run the left and right hydraulic systems and power the flight controls. Therefore you need the RAT to power the center system to saisfy the regulatory agencies (and most pilots) that the airplane can be controlled after a dual engine failure.
In contrast, there is no RAT any 707, 727, 737, DC-9 or MD-80/90 because all depend on manual reversion if all engines fail.