No problem. Yes, Mangano was also a Maranzano ally. But Maranzano settled on Scalise to run the old Mineo Family. According to Valachi (and I think also Nicola Gentile mentioned this), Mangano´s name was put on a "death list" with Luciano´s, Genovese´s, Costello´s, Capone´s and Dutch Schultz´s by Maranzano. So by September 1931, Mangano had clearly fallen out of favor with Maranzano who gave the murder assaignment to Scalise. But Scalise balked and informed Mangano of the murder plot instead. At the same time, Lucchese (another Maranzano ally) informed Luciano of the "death list" and from then on Maranzano was history.
It´s hard to tell why Scalise was demoted after the Maranzano murder. Joe Bonanno referred to the incident (in his book) only as "Scalise´s star fell. Scalise had been too close a supporter of Maranzano. With Lucky´s rise to power, Scalise became a liability to his Family, which didn´t want to antagonize the powerful Luciano and his cohorts."