I'm not sure of the exact day and time when Rothstein first hired Lansky and Luciano but it certainly was in the very early 20s. IRL everyone who knew Lansky at that time remarked on his relatively studious nature compared to people like Siegel and Luciano.

Rothstein had people not only like Luciano and Lansky on the payroll but also folks like Dutch Schultz and Legs Diamond. As a bootlegger and later narcotics importer it stands to reason that he would have had enough manpower to at least give others pause before taking something he thought was his.

The war being shown is not between blacks and the mafia but rather between two , really three factions of white organized crime with a small black segment throwing in with one faction.

Enoch Johnson also never killed anyone, let alone a virtual foster son. The show is not a documentary. I agree that the amount of violence is over the top but that's entertainment.

As mentioned given that the twenties had seen recent race riots where blacks killed whites and vice versa and at this time there were independent black criminal operations it's not beyond belief that a Chalky White would exist. Resisting the Outfit's takeover of numbers in the 50s a black man killed one of Giancana's aides. Nobody is bulletproof no matter which organization they belong to.