The idea that Lee Harvey Oswald was, at the same time,
a) a CIA agent working to develop a poison to cause cancer in Castro and smuggle it into Cuba via Mexico
b) the designated patsy in the JFK assassination, set up up to be the lone gunman
is quite preposterous because it is inconceivable that they would use him in both ways.
First, note that these two missions were not of equal importance. The assassination of Kennedy was infinitely more important because it was effectively a coup d'etat - an overthrow of the existing government of the United States of America.
And making it work (Oswald as fall guy) was of crucial importance because if it failed- if they couldn't implicate Oswald for one reason or another- then the exposure of the true nature of the crime would have been instantly transparent. Who was going to believe that the man or men who killed Kennedy got clean away like a fart in a high wind?
So, considering everything that was at stake in killing Kennedy and blaming Oswald, WHY would they, at the same time, also involve him in a motley crew, Manny Moe and Jack kind of scheme to kill Castro where they were mixing up poisons in somebody's bathtub and trying them out on mental patients? Come on. The CIA could do better than that. And frankly, they didn't need Oswald for that. What did he know about it? What did he have to contribute to it? What aspect of his training qualified him to be involved in it?
Remember, Oswald had just spent three years in Russia where he worked at a radio factory. And since returning 10 months before, he held several odd jobs in Dallas, none of which worked out. So what qualified him for such an operation? Why would they choose him for that? Wasn't just about ANYBODY ELSE just as good or better than Oswald for that?
And remember: they weren't setting him up to be the CIA agent who killed Kennedy. They were setting him up as the disgruntled, resentful, Castro-loving, socialist, communist, Marxist who killed Kennedy. So, why would they involve him in another CIA operation to kill Castro which, if exposed, would destroy everything they were doing with him in regard to JFK? It was a risk that was not only not worth taking, but which offered no rewards. Whatever practical value Oswald had to the other operation could surely have been provided by: ANY OTHER PERSON ON EARTH. So, why use Oswald for that?
No, on the contrary, once Oswald was chosen to be the Dallas patsy, then everything they did in relation to him, including all the things they did to steer him, were done in light of the Dallas plot. Nothing else. Was it that big? YES! Of course, it was that big. They don't call it the "crime of the century" for nothing. It was the overthrow of the elected government of the United States, followed by a reversal in the direction of US foreign policy, including an escalation of the Vietnam War. Some plot to poison Castro with a cancer-causing agent was NOTHING in comparison. How many plots and schemes were there to kill Castro? You couldn't count them all. They weren't going to risk anything in the JFK assassination just to get Oswald's services for the other. They didn't need his services for that. I'm sure they didn't want his services for that.
The whole story of Oswald being involved in the other plot needs to be categorically rejected. To the CIA, Oswald was their patsy; that's all. It's all he EVER was. Even when they sent him to Russia, he was their patsy in Russia. He was never anything but their dupe.