does the x52 pro DEPEND on the throttle to work - No
can I just use the joystick and a bank of foot switches, voice attack or similar and an analogue pedal for the thrust? - yes
Is every single button and switch on the X52 redefinable? - yes
Well, strictly speaking the joystick does depend on the throttle in as much as it can't possibly work without the throttle being plugged in. The joystick attaches to the throttle by a PS2 cable, and the throttle connects to the PC via USB.
But you don't need to use it if you don't want to, you can remap all throttle behavior to other keys/peripherals.