The chemical weapons convention entered into force in 1997, so the majority of attempts on Castro occurred before the Chemical weapons convention entered into force.
While assassination may sometimes be legal during a war, it is not legal outside of hostilities. One is allowed to kill enemy hostiles. One is allowed to use lethal force against someone who is threatening your country. But you can't kill someone who isn't an imminent threat
The assassination of Castro would be an extra-legal act, if it occurred outside of hostilities. And if the USA were defeated in war by a state friendly to Cuba, former presidents could be in trouble. However, the USA has the best-funded military in the world.
The chemical weapons treaties envisage the use of chemical weapons in war. There is not much if anything specifically about the use of chemical weapons in peacetime. It is not a radical interpretation of the convention that it doesn't intend to ban chemical weapons in war, but allow them in peace.