Hydro Mechanical Unit
Skytrucker,
Even without 737 knowledge, I can make a start answering your question.
Details will then have to be filled in by a 737 skilled person.
First, what we call the HMU on 'my' aircraft, the 747-400, does not have brains by itself.
The brains are inside the Engine Control Unit, which is a large electronics box located on the engine's fan case.
The HMU is only the 'hand and feet ' of the ECU and converts electrical inputs given by the ECU into fuel pressure outputs to control the engine's fuel flow and air control components. (bleed valves and variable stator vanes).
When a failure occurs inside the ECU, the unit switches from channel A to B or vice versa. If that does not give impovement, the unit switches to alternate mode.
This means that it will calculate with assumed environmental temperature and ambient pressure.
This mode will cause more fuel consumption, but is still capable to protect the engine against overspeed or high temperature.
If the ECU fails completely, the pitots will have to switch to HARD mode manually, which indeed gives direct input from the power levers into the HMU.
Then the flightcrew have to monitor the engine speeds by themselves, since no protection is offered anymore by the ECU.
I hope this story will answer question #2 as well, as you may see that with the APU running, the engines do not have to deliver bleed air and so can produce more thrust instead.
The ECU (not the HMU) then regulates the engine to max RPM or max EGT, which ever comes first.
Bye,
Ballpoint.