to add a non-pilot (burn him, burn him, but sometimes the ignorant can ask relevant questions..) question to the mix.
Could the reason that on both fatal flights that we saw unexpectedly brief presses to the "trim up" pickle switch be :
because that the pilot would have expected that his pressing of the switch to cause the immediate cacophony of noise of the trim wheels spinning , and maybe they didn't due to aerodynamic loads currently on them due to the lack of authority of the electric motor due to increasing speed so they quickly released the switch ("It's not working") . This would have been then followed up by MCAS throwing in it's "i'm OK to go again jack" logic of another bucketload of nose down. And the same side effect of increasing speed could cause the inability to wind back manually leading to the desperate measure of turning the electronic back on.
I guess the question is - is there a combination of speed and nose down trim that can mean that neither the electronic trim nor the hand wind trim can counteract it. And of course if you re-enable the electronic trim to try, having manual trim try and failed, then MCAS gives you an extra dose of "nose hard down"...