Cerone was never Giancana's underboss. Giancana didn't like or trust Cerone. Accardo did not have the power to exile Giancana. Giancana voluntarily went to Mexico and other places outside the U.S. and made a lot of money. It was time for him to go because of all the heat. Accardo had no power to murder Giancana. It was the other way around. Giancana was the Boss of the Taylor St. Crew which was the largest and dominant Crew in the Outfit. Accardo was connected to Elmwood Park and was a Top Level Advisor who was semi-retired. Ricca was the top Advisor and kept the peace. Accardo never ordered any murders without Giancana and Ricca's say.
Elmwoodparker, you clearly have a lot of knowledge on the Outfit. However, I disagree that Accardo did not have the power to exile Giancana or to have him murdered. The fact is Accardo survived and Giancana did not. When Giancana got clipped, Rica was dead and Accardo was the most senior guy in the Outfit. I refer you to Gary Martin's post earlier in this thread dated 2/3/2013.
While we cannot know all of the facts, and everyone is entitled to their opinion, I think Gary's post summarizes the general consensus and historical record.