If this guy is loosely based on Jack Warner and Johnny is supposed to be Frank Sinatra, can we assume that Warner actually received an "offer he couldn't refuse" from the mafia? Is this how Sinatra really got the part in From Here to Eternity that revived his career?

I also wonder who the starlet is that Woltz refers to that Johnny stole from him and ruined. Ava Gardner?

A think they were just loosely based on those people, not to the extent where the things in the film actually happened in real life

Jhonny Fontane is based on Frank Sinatra, yes. The story of the Bandleder were Micheal talked about on connie's wedding (with Kay)is also real. Willie Moretti held a gun to the bandleaders head. He signed for 1 dollar. In the godfather they say 1000 dollar.

That's all i know.

There is a real-life parallel to the Woltz episode, but it's far less dramatic:
Sinatra's career had been declining in the Fifties. In 1954, he desperately wanted the part of Maggio in "From Here to Eternity" as a way to get his career back on track. The real-life studio head who initially opposed Sinatra for "From Here to Eternity" was Harry Cohn. But Cohn was never muscled by the Mafia, nor was he opposed to Sinatra because Frankie stole his gal. He wanted Eli Wallach because Wallach was under contract, but eventually chose Sinatra because he was skinnier than Wallach, and thus fit Maggio's description.

Thanks, good info.