It's also necessary to understand that the FM is not intended as a 'how to fly' book, nor will it contain everything you need to know to fly the machine. Lots of stuff that would be useful is not in the 'approved' section, as it doesn't need to be to be certified. Range and Endurance information, if it appears, is in the 'manufacturer's data' section and is not subject to any scrutiny by the FAA or any other certification authority.
Getting more info in the HV section is, sadly, wishful thinking. The FAA isn't going to demand it of a helicopter that was certified more than 40 years ago, and Bell isn't going to provide it for (pick any one of the following)
liability reasons,;
cost of obtaining the data reasons;
usefulness to operators reasons;
and so on.
As for the 'real' engine failure being different than the deceleration of the engine with a throttle chop - during the certification program, the airframe manufacturer would have to show the FAA that the engine deceleration was similar enough to a real engine failure as to be representative- if not, they would have to demonstrate at least one HV point with a 'real' engine failure to show similarity. (Pretty gutsy point to do). But it would be done.
We had to use the training mode switch on the 427 to simulate engine failures, having proved that it was identical to the real engine failure, and that throttle chops were nowhere near the same as the real thing.
So it does happen.