Grav,
Congrats on doing the studying!! (BTW Chuck Keys was on the Comanche team) And on really understanding the issues:
Nr increase with speed or maneuvering - right on, Comanche set its rotor speed automatically based on maneuver state, see patent number
4,998,202 at http://patft.uspto.gov/
The reason why the hover efficiency increases with reduced solidity (or increased Ct/sigma) is that the profile power is too high when a big blade is hovered. Things want the blade to work more than a wide chord blade must work in the hover, so skinny blades at more angle are better. See in S+K where the Figure of Merit is plotted vs Ct/sigma and note that the Ct/sigma wants to be about .125 or more at peak FM. Since most high speed helos want to have hover Ct/sigma at about 0.07 to .08, that means they throw away a significant rotor efficiency potential (about 7% thrust at equal power.)
Why not beep down in a hover and up in cruise? We do, the S76A allows that. But not all helos can work at wide rotor rpms, vibration, tail rotor thrust and shaft critical speeds all need more work than it is worth for some designs.