Just to elaborate on #3.
The different driveshafts are ET (engine to transmission), mast, and TR (tail rotor).
Twin engine helicopters can either have a single driveshaft between a combining gearbox and the main gear box (e.g. Bell 412), or a separate drive shaft from each engine to the main gear box (e.g. S76).
Single engine helicopters have a single driveshaft between the engine and the main gear box.
If you only have a single drive shaft to the main gear box, and it fails, you have 2 issues. Loss of drive to the main rotor, requiring autorotation; and if that engine/s doesn't have an electronic overspeed system it will explode (likely uncontained engine failure). Not that the engine was helping you in any way once the driveshaft failed. So you could have a fire, severed electrics, hydraulics, other damaged components etc etc
If the mast fails (the driveshaft that the main rotor connects to), you'll arrive at the scene of the accident as a passenger.
If the TR driveshaft fails, it doesn't matter how many engines you have, without drive to the tail rotor you'll be shutting down all the engines very soon and executing an autorotation.