When Diabolik steals the lorry the driver runs after it. Diabolik then drives it quite a distance before he jumps off. After the lorry runs off the road the driver stops to look at the accident. Then Diabolik and Eva drive past the lorry driver without him having noticed that Diabolik had run back to the car.
Crew or equipment visible
Diabolik decides to hide the 11 emeralds by removing rounds from the magazine of his sub-machine gun; replacing each bullet by a gem, load them back into the magazine and fire them into Valmont. Since the emeralds are long and too wide the complete assembly would not fit back in the magazine. Even if it did the gem's cross section does not match that of the bullet so when the shell is fired most of the hot gasses would flow around it rather than pushing it out of the barrel. Finally, would the emeralds even have withstood the stress? It would have been far safer to just bury the emeralds, shoot Valmont, and retrieve the gems later.
The picture that Diabolik mounts in front of the surveillance camera during the emerald heist would not be able to substitute the actual picture on the police monitor. Firstly it is taken with a hand-held camera without flash, and secondly after mounting it is only lit from the back and would therefore appear much darker on the monitor.
The gold ingot is lowered into its "coffin" using simple rope. Two strands of rope of this thickness would not be able to handle anywhere near 20 tons. (The lid of the "coffin" is however lowered using steel cables though.) Also gold is quite soft and flexible and should have more underneath support than what is given in the film.