I found "Once upon a Time" legal and for free on Dailymotion: https://www.dailymotion.com/video/x7uv75b

Alas, it was not the movie I was looking for. ?
Nevertheless, thank you, guys, for you help! Maybe it's time to admit to myself that I will never find out the name of the film I was watching as a teenager. ????
If anyone knows a mafia flick where a victim is getting gut-punched and a pistol put into his mouth during an interrogation, and where a Monroe-esque showgirl is being shot in the stomach thrice by an old mafia boss, please let me know!