I do find it plausible that the mob had a hand in the JFK assassination. The Kennedys happily accepted assistance from the mob in getting JFK elected, and then double-crossed them. I suspect old Joe would have advised against RFK’s attack on the mob had he not been incapacitated by a stroke. Old Joe was a scoundrel, but he wasn’t stupid.
I also found the book DoubleCross to be very believable. I believe the mob was involved in some way.