Certainly two men armed with machine guns could only be overcome with a knife under one set of circumstances: they had not to suspect that the men they were approached by would kill them. So they had to have met on a prior occasion. And we know also that if a knife was the murder weapon, there had to be at least two men who managed to strike the assassins at exactly the same moment. "Difficult, not impossible."

Just because they turn up in a ditch with their throats cut (you can see this in the film, you don't need the screenplay for that) doesn't mean that that they were not shot. It is not out of the realm of plausibility that they were shot first and then had their throats cut for insurance -- stranger things have happened than people surviving serious gun shot wounds, but a deep slice from ear to ear, no way (Fanucci's attackers are said in the novel to have deliberately given him a shallow cut just to scare him). Sure, we don't hear further shots, and I'm not sure if the silencer had been invented at that time (a towel or a big potato on a gun barrel would have given the assassins' murderers away), but this does not necessarily disprove the notion that the assassins were shot.