You know how people say it would be impossible to cover up something so big? It would have to leak?
Well, it did. The CIA/Mafia/Cuban assassination connection is firmly established fact now. Among many other important facts use to be "conspiracy theories."
Motivations tl;dr https://i.imgur.com/WEjm2ig.jpg
Goodness me, no. Have you read the sources Oliver Stone borrowed from when cooking it up? To say they are fantasies would be giving a disservice to the word.
Odd choice of countries.
They were in the news around then because of possible attempts they were making to break away from Soviet control. Yugoslavia had already broken with the Soviets and in 1961 setup the non-aligned movement[1].
In 1956 the Poles had refused to vote against the UN resolution condemning Soviet intervention (invasion) of Hungary and there was some hope that these splits could have been exploited.
This isn't entirely impossible. In 1970 Poland and West Germany signed a treaty, which to some degree had been brought about by the Catholic church (which had been suppressed by the Polish goverment in the early '60's). Counter-factuals are hard, but Catholic JFK may have been able to change things earlier, there.
Of course some couldn't see any distinction between the Poles and the Russians ("all commies") and so you get posters like this.
[1] https://en.wikipedia.org/wiki/Yugoslavia#The_1948_Yugoslavia...