This is another plot gap between II and III:

Michael certainly would not let the Olive Oil Business simply slip away in '58 just because Frankie betrayed him.He needed it. He'd appoint a successor immediately (although I understand why FFC didn't want to make a very long movie longer by showing that succession). Zasa would have been way too young (he looked to be about 40 in '79). Altobello wins by default among some posters here. But, Altobello would have been a contemporary of Vito's to have been honored by becoming Connie's godfather--no way would Vito have bestowed that honor on an up-and-coming button man in his 20's. And, Michael treats Altobello too respectfully for him to have been a subordinate to hyper-controlling, contemptuous Michael.

I don't have any better idea of who Altobello was, and why he turned on Michael, than I do of why Michael wanted Immobiliare.