The fact that electric trim is still functioning does not indicate that it is not the cause of the runaway. In fact, the way the NNC is written, it seems that it would be expected for electric trim to be able to counter a runaway in many situations. The manual wheel/manual trim system has priority so the fact that the electric trim worked would indicate that the manual (non-electric) part of the system wasn't the likely cause.
In the electric trim regime I see 5 possible failures that lead to run away trim:
1. The manual electric trim inputs have a short or are stuck or any other failure
2. The actuator fails and just trims till the max/min
3. The wiring fails/shortcuts
4. The computer fails
5. The manual non-electric trim fails
Now how does this impact the pilot, or how is it felt:
1. The trim is continuous and can not be stopped if smacking the control column does not release the switches. Nothing can be done except hit the cut out switches
2. Will be a continuous trim that will not stop if you flick the thumb switches
3. same as 2.
4. This is actually the tricky part because the manual electric trim does still work and if activated will kill the signal from the computer because it has priority over the computer
5. This would be a death sentence because no matter what both ways of trimming would be tarnished and there would be no real way to use electric trim if manual is prioritized over electrical
Now for Point 4:
If the computer fails, or in case of MCAS actually does what it supposed to do (trim with unlimited authority no matter what as long as AoA>X) then it does not present itself as a runaway because:
1. Electric trim does still work and
2. After you did trim with electric trim there is no immediate
restart of trim caused by the computer, there is a 5 second break. This actually means that the trim action is deliberate by the computer and not a malfunction.
The problem is that it was never stated that a false input from the AoA probe will lead to step-wise
nose down trim.
The problem could also be a broken feel computer. At the end it is, with no knowledge of the engineering and function of the software, almost impossible to conclude that a broken AoA sensor leads to the computer deliberately apply step-wise nose down input.
The real problem seems to be that a broken AoA probe leads to a cascade of problems in the 737 without ever telling the pilot that the probe is broken. The trim did not run away or had a failure, all the trim inputs from the computer were legit and working as intended. From the aircraft perspective only a full nose down input was the solution to a problem that actually was not there.