you are correct. the toxin from a stingray barb is thermolabile. immersion in hot water over 100 oC for 30-45 minutes will inactivate it. as it's a proteinaceous toxin, it will denature unde high temperature, just as an egg white gets cloudy when you cook it.
the rational behind papain is that it is a cystein protease. i don't know if the primary (amino acid) structure of stingray toxin has been determined. if there are not a lot of cysteine residues critical to the activity of stingray toxin meat tenderizer (papain) may just destroy sensory neurons locally at the site of the sting and attenuate the pain felt from the sting.
sorry you did not receive first world care in the third world.