How does the mechanical backup on the 320 work? + some questions

[Reimers]

The RAT provides pressure to the blue HYD system, which in turn can then drive the emergency generator. It does not provide elec power itself.
On a large fly-by-wire aircraft, you usually require hydraulics to move the flight controls and electronics to tell the hydraulic jack to move.
There is no mechanical back-up for hydraulics on the fly-by-wire arbusses.
There was a mechanical back-up for the electronics associated with the rudder and the THS (trimmable horizontal stabilizer), but newer A320s have an all electronic rudder.

So, if you have no hydraulics (3 of which you started out with), you can only fiddle with the engines to alter the way the aircraft goes.
And, if all of the 5 (A330/A340) of 7 (A320) flight control computers fail, you are left with the engines and, perhaps, the mechanical back-up.