I know this thread's a little old but great discussion. Don't believe I've seen it before. Thanks for the current link, GangstersInc.
I think Marcello and the mob were used to play a role in the assassination, but were not the top conspirators. My own personal stretch theory is that the conspirators' intention was to use the mob as a scapegoat, either instead of Oswald or along with him, and that Nixon was influential in burying that plan due to the mob's help in his rise to power. So, Oswald became the Patsy.
The Godfather 2 is a great movie, but the narrative that the mob played the U.S. government was fantasy. It was the other way around. It was nefarious factions of our government that used the mob and then disposed of them when they felt they weren't needed anymore. Interesting that the RICO act wasn't truly implemented until years after it was signed into law. They knew about the Commission when Valachi testified but it wouldn't be until the 80s that a fed prosecutor would mount a commission case? Clearly there was still a lot of juice to be squeezed from that lemon.
It's still uncertain as to who killed Joe Colombo, btw. His politics may have pissed off Gambino but they really pissed off the government. That first Italian civil rights rally had half the city in attendance. Like with Kennedy, nobody had more to gain from Colombo's death than the government.