Yes, but not necessarily for the reasons you have here (more later).
First, Castro was an obedient lap dog that did not do anything without the Russian’s permission.
Uh, not so much as you think. Note that only 13 months earlier, Fidel damned near started WW3 all by his lonesome. He was sitting in a Soviet AA missile battery during the Cuban Missile Crisis when a US plane appeared on the scope. They watched for a couple minutes as it approached Cuba and then Castro asked if the big red button would launch the missile. The Soviet military guy in charge said yes, and (to everyone's shock and horror) Fidel then went and pushed it - launching the missile and shooting the plane down. Not exactly an obedient lapdog, more of an impulse-driven, rabid little terrier.
The Russians were actually a lot smarter and a lot more aware of our political system then people thought. They knew Kennedy’s position on civil rights was creating strong headwinds. They also knew that Joe wasn’t in a position to buy the election as he did in 1960. Killing Kennedy really made sure his policies would continue, and they knew that.
Yes, they were astute observers of our system. However, you were dealing with a sitting President and while some of his opposition was strong, it wasn't so numerous as to seriously endanger his re-election. At that point, no President had been kicked out of office for 31 years, and that required the Great Depression to bring it about.
I don't think that the Russians did it, for a very simple reason: if found out, it would have guaranteed WW3. The entire free world would have ganged up on them in an instant, and we had an enormous lead in deliverable nukes. Russia simply couldn't afford to fight a war, especially not after having been bled white by the Germans only about 20 years earlier.
One of the great mysteries I have always wondered about is “What if Kennedy had lived?” I was only 13 back then, but I lived in South Texas and remembered the talk.
While he was young and charismatic, many of his policies were NOT popular. And those policies had made a LOT of enemies. Could that have translated into an upset? We'll really never know.
Once he was killed, people I had heard cussing him out just a couple of weeks before, now all spoke of him as a saint. Like most 13 year olds at the time, I thought he was great - but then we didn't vote.