I have a general comment and some questions about functions (2) through (4) in your first post.
The comment is that one needs to be careful about providing too many distinct warnings, or having too much confidence in how the pilot will react to them. In an extreme case, one of the recent 737-MAX reports suggested that crew reaction was not what Boeing or FAA expected, and perhaps that is worth considering here. Granted, one is unlikely to get multiple warnings at once to confuse the issue as can happen in a complicated airliner cockpit, but one must still process which warning it is and what should be done about it. Maybe there is virtue in keeping it simple and not providing too many artificial cues for too many situations in the same device (physical cues being already provided by the aircraft and instruments), for easier and more reliable response by the pilot. I think there may be merit in solving one big problem well without trying to solve them all, from a human factors standpoint.
Now the questions:
As to (2), "behind the power curve" flying, one will be in the region of reversed command whenever slower than minimum power required speed, including slow flight at altitude, vertical sinks, and so forth. It's really only a problem when very close to the ground with the intention to climb, as in the FAA PTS tasks for "liftoff at low airspeed and high angle of attack" (the pilot-candidate is expected to detect the development of that condition and take prompt corrective action). I would find a warning under any other flight regime to be extremely annoying, especially if it seemed to come on too often. Áre you planning any sort of above-ground altitude input for this? Were you planning to address high sink rate conditions at altitude with this as well?
For (3), is this just an aural / warning light addition to the normal function of the lower arc of a g-meter?
As to (4), I question how one would get into a situation with simultaneous high mu and "low" rpm. At retreating blade stall, the rotor rpm will indeed be low relative to the forward speed, but not low in any other sense and should be expected to be noticeably above normal cruise rpm (assuming positive g-load; otherwise we're in one of the other situations). I've never gone all the way to retreating blade stall in a teetering rotor gyroplane (if I interpreted his comments properly, I believe Mr. Beaty has suggested elsewhere that this would amount to a benign stick full-forward situation before any catastrophe ensuses) but in the A&S18A one gets increasingly uncomfortable vibration that provides ample warning, and no sudden departure from controlled flight (helicopters can produce a pitch-up that is a speed-reducing and thus a self-correcting tendency). In short, I don't really perceive a problem needing a cure here. What is believed to have gone wrong in the accident you mentioned?