I can't see Michael abandoning the Corleone operation in New York, his muscle
No, indeed. Mafia families, like nature, abhor a vacuum. Had the Rosatos killed Pentangeli and
not gotten into that shootout with the police, they would have stepped in and imposed themselves as the new leaders. Probably the remaining members of the NYC Corleones would have accepted them just so they could get back to earning money and to avoid an internal war. But, although Michael thought Pentangeli was dead, he must have known right away about the shootout with police, and that the Rosatos, consequently, were on the run. So, I'm
guessing, he would have dispatched either Rocco or Neri to NYC immediately to restore order, and to vet another, temporary, caporegime to report to Michael. (I'm guessing he sent Rocco. The novel says Rocco stayed behind to back up his
padrone, Clemenza, when Michael moved to Nevada.)
It seems to me Michael appointed / elevated, the “talented†Joey Zaza, the long standing member going back to Clemenza's old regime and Michael was running the Corleone business from Tahoe by proxy until Zaza was officially awarded the business
If Zasa was about 40 years old in 1979, when III opened, he would have been 19 in 1958, when the Rosatos attempted to kill Pentangeli--far too young to even have been "made," much less a "long standing member" of Clemenza's regime.
The best I can do to answer my own original question is that, per the above, Michael immediately appointed another (unknown to us forever) caporegime in NY. Zasa began his rise through the ranks. Some time after that, when Michael sold the casinos and no longer needed NYC's muscle, he turned it over to Zasa, thus completing his journey to "legitimacy."
