More fascinating than ever. There have been some guesses here that are now proven to be very close to what actually happened.
Without attempting or wishing to accuse/blame the pilots - I wonder if what we have here is a tragic misinterpretation of events?
The Captain returned to the cockpit, but did not feel it vital to assume control - this surely suggests that as far as they believed, the situation was under control.?
Could the pitch up commands have SIMPLY been in response to a belief that the descent was due to being nose down - as others have said - to pitch up BELIEVING that the aircraft is stalled is surely the last thing that you would wish to do - conversely if you believe airspeed is OK then you would naturally 'pull up' to arrest descent.
This theory does not take account of what, if any indications the pilots were (or not) receiving - they either beieved the stall warnings (when they came) or did not. It is hard to believe that any pilot would make a control input (even if it is wrong) UNLESS they believed it was the correct course of action.
So the nub is: simple mistake?