There can hardly be any doubt that the spin you refer to is a product of the downwash. After all if the survivor was similarly suspended from a hangar roof with the doors shut you would not expect much spinning to arise.
Clearly any rotor downwash is not a very steady stream of air and will vary with helo type, local wind and ground effects. To suspend anything in such an airflow and expect it not to rotate is pretty optimistic.
Since the velocities involved are modest could I suggest that what you wish to study might be looked at usefully in a pretty simple ‘wind tunnel’/lab experiment with a simple ceiling mounted fan where you could well come up with some useful data on the way various shapes of suspended loads react to a known airflow asymmetry? I say known because you will be able to measure, control (and even visualise) the airflow you provide relatively easily in the lab.