From someone who's certified helicopters and taught certification techniques.
A couple of points on the HV curve, in the hope of clearing up some misconceptions. For civil helicopters:
For any helicopter with more than 9 passenger seats, the HV curve is a limitation. If it's a twin engine helicopter, the HV curve is for a single engine failure. Less than 9 passengers, the HV curve is in the performance section.
All HV curves have at least two 'parts'. The part from the low hover point to the knee is flown using takeoff power, but no intervention time from engine failure to pilot corrective action. The part above the 'knee' is flown using power for level flight and a one second intervention on the collective before any action is taken.
For the low hover point - the collective only allowed to be raised. During the HV demonstration, you can't lower the collective and then raise it later.
In summary, the HV curve test conditions are not, repeat not valid for an approach and landing situation. The only place you're going to see an engine failure during the approach and landing situation taken into consideration is for Category A profiles.
And if you want more guidance, look into the Advisory Circulars for Part 27 and Part 29 helicopters.