The only way Dellacroce would have became boss was if he made a move and took it by force.

I don't think he had the overall support of the Family. And I am convinced Carlo Gambino would have never made him his successor.

It's not just because Big Paul was his cousin. Neil was an Anastasia guy and there's no chance Carlo would have chosen him. Carlo Gambino made him his underboss because he knew he had to put an Anastasia guy in that position in order to keep the Family's unity and Dellacroce was one of the few Anastasia guys he could deal with.