Organophosphates are very dangerous chemicals but they are not related to mustard gas. So far as I know, mustard gas is made from ethylene and sulphur chloride - no phosphorus. Chlorfenvinphos and propetamphos are both organophosphates and contain phosphorus.
While it is almost certain that people using or being exposed to any of those substances might be poisoned - both acutely and cumulatively (OPs are gradually metabolised and broken down in the body, but chronic toxicity can occur) the two OPs are schedule III poisons: "Rubber gloves, a coverall and face-shield must be worn when handling concentrate."
Other objections to the letters are numerous, but I will cite only one. What evidence is there of BSE being caused by OPs? Scrapie was present in sheep 40 to 50 years before OPs were invented.
A L WINFIELD