Green Dot
"from instrument indications, due to loads, the crew on the flight deck would have been thrown violently forward and to the left or right, depending on direction of the flat spin. Perhaps unable to move at all for periods of time if the rate of spin was high enough."
Why all the violence? Surely there is enough in the turbulance to satisfy your worst nightmare? In a spin the bigger the airplane the less violently it behaves - to a point and, by the way, centrifugal force builds in a flat spin with both wingtips pulling towards the horizon, which is why it is a spin and not a continuing roll.
If you think about what you wrote there is little difference between what you say and what might be found in severe turbulance - I might submit that you could stall an airliner and enter a spin, in conditions of severe turbulance, without even noticing any change to your discomfort and the real point is that there is little, if anything, that will tell you something else has happened - except, perhaps, systems will start shutting down.... including engines, at the high angles of attack associated with the situation. So it is not impossible that some of this might be passed along via ACARS and much of it... not.
Ciao