
If that is crazy, I'm insane...
Hmmm, will this idea end up in cinema??
Your plan has a small problem: you will have to show all the disasters that will happen if they don't save JFK or there is nothing to show. (Like Terminator 2: you need to feel the tension of a 'darkness coming', before you can feel the relief it didn't happen.)
And you will have to show why and how these things/events are connected.
Nobody will believe it when you stop the moon from falling from the sky by saving 1 butterfly (just to name a thing).
Your setup sparks an idea:
Save Lennon and let him appear in the future as a desillusioned man without inspiration; his days of fame are long gone and his star has faded. Yoko has left him. He is still touring pubs and wonders why he got saved that day... Hendrix, Jim Morrison, Kurt Cobain and Elvis died early and are still legendary... he isn't. Did god punish him for saying the Beatles were bigger than Jesus???
When the heroes appear and tell him they saved him but traveling back in time he wants to go back and stop them saving him....
Not a real comedy, but it could be a funny and sad drama...