My personal take is that Lombardo took over as Acting Boss around 1965/67, as it seems to suggest in one of the Mary Ferrell documents, where he is described, by an an informant, as being appointed head of the whole of Cosa Nostra by Genovese.

Some hyperbol there no doubt, but I believe that was when Catena pretty much retired to Florida and Eboli took over as Benny Squint's underboss/acting underboss. I have seen some suggestion that Catena's retirement was one of the subjects to be discussed at the aborted LaSella restaurant meeting.

When Genovese died in 1969 I think Lombardo was made Boss simply because he had the support of most of the more powerful Captains.

Just to add, I don't think there was any real intention to set up a front Boss, initially at least. I simply think that Eboli was the underboss and carried out many of the Bosses functions, as Salerno did for Gigante later, because Lombardo liked to keep a very low profile.