4 If the runaway continues:

      STAB TRIM CUTOUT switches (both)   . . CUTOUT

      If the runaway continues:

        Stabilizer trim wheel    . . . . .   Grasp and hold

  - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

  5  Stabilizer    . . . . . . . . . . . .   Trim manually

This is what the Lion Air crew of the flight before the one that crashed did, in response to the same sort of MCAS failure, and completed the flight without further trouble. While Boeing made a serious error in hiding the differences between this variant and its predecessors, it seems that the prior trim runaway procedure does work for this sort of MCAS failure (unless evidence to the contrary comes out of the Ethiopian Airways investigation.)

I think the problem is that the symptoms are different from other 737s, and if you don't KNOW about the MCAS stuff, that's very surprising. So as humans, even though logically the same procedure applies, instinctively you don't think to do it. And given the very short time frames involved, and the margin of error, it's not too surprising that these events have occurred.

Perhaps the need-to-retrain criteria needs to include the following amendment:

If the craft exhibits new symptoms, even if the response to those symptoms are covered correctly be the old operations manual, pilots will need to be retrained to account for the new symptoms.

No, it doesn't and that's the problem. A runaway won't stop when you hit the trim buttons.

Depends on what causes the runaway. If the autopilot goes berserk, the trim buttons will override it (if I recall correctly, it's been a long time).