The airplane’s major structures were all accounted for at the accident site.
So no breakup during flight or loss of any major structural parts. A corkscrew path sounds suspiciously like a spinning airframe. The instructions for a stall test state 'not above 18,000 ft' but they started at 20,000 amsl. Those 2000 ft should not create a major problem all of a sudden, but the meteorological conditions could have led to a higher true altitude. Just guessing here as I cannot remember the formulas right now...