Nick,
All the point you make have validity, with one exception. At the risk of belaboring a point, I do not believe that a 'rotorcraft' can achieve significantly higher forward speeds without having some configuration of twin counterrotating main-rotors.
I think you will agree that high forward velocities necessitate a reduction in the rpm of the rotor(s) so that compression at the advancing tip does not become a problem. This high forward velocity combined with the slower turning rotor(s) puts a significant portion of the retreating side of the rotor into reverse velocity.
Mart suggests simply feathering the retreating blades, however, logic, and others, suggest that this reverse velocity should be utilized by putting the blades on the retreating side into a negative pitch.
Two years ago Sikorsky proposed that high forward velocities could be achieved with their single rotor
Reverse Velocity Rotorcraft. However, the forgoing linked web page gives a strong and valid reason why I believe that the single rotor proposition is fatally flawed.
IMHO, Sikorsky's recent return to the twin-rotor configuration reaffirms the necessity for the lateral symmetry, which is provided by twin main-rotors through the complete range of forward speeds.
Dave