Heart attack, suffocation, strangulation. Botulism severe enough to cause respiratory muscle paralysis or - interrogation/torture using curare like medications where they failed to keep him properly ventilated.
I agree. Whagree.
I don’t believe they would have used him as a testing subject in the way you explained. Not when they have thousands in their camps they do that sort of thing with all the time. Why would they damage the merchandise they detained/kidnapped for the deals they would be demanding for his return??.....