If you were to post this on 'rotorheads' I think you'll get loads of answers; some of which may be correct!
To answer the basic questions:
1. The curve is in the FLM because it has to be according to certification requirements (see following extract from FAR Part 27)
"27.1587 Performance information.
(a) The rotorcraft must be furnished with the following information, determined in accordance with §§27.51 through 27.79 and 27.143(c):
(1) Enough information to determine the limiting height-speed envelope."
2. There is no strict relevance to operations in PC1 or to Cat A standards, as the procedures developed for such operations specifically avoid the HV curve and are detailed in a separate FLM supplement.
3. Non Commercial Air Transport operators can operate to whatever PC standard they wish, but the FLM part 5 will normally describe the Cat B take off as being required to avoid the HV curve. Naturally some knowledge of this is useful for u/s load ops or camera work etc.
4. The normal, single graph, HV curve is only relevant for a specific Density Altitude and weight, and for level flight, though some aircraft types have a series of graphs available (such as the B-212 FLM) where more detail can be obtained. Normally during landing the HV will shrink and may even disappear. At some lower weights it will disappear too, hence the use of vertical Cat a procedures.
As I say, someone like Shawn Coyle or Nick Lappos will be able to expand on the subject.
cheers