In a conventional helicopter lift is shared by either side of the rotor, flapping to equality. As the airspeed goes up more and more of the retreating blade ends up going backwards and producing no lift leaving a small section to produce the required lift, when this becomes too much of a burden it stalls.
If the inner section could be made to produce lift then the outer section would not have to create so much lift hence delaying or preventing retreating blade stall altogether.
Wider blades may be necessary to carry all the control gubbins, as these rotors are clearly not going to work conventionally. In general wider blades reduce efficiency, but I guess the benefits of higher speeds must come at a cost.
I can see how the hover will work and can see the high speed bit may be feasible, but how it gets from one to the other will be interesting. And they said the V22 has interesting transitional problems!
In answer to the two questions an autorotating rotor would definitely hinder high speed flight and while retreating blade stall is possible on a driven rotor I think not here. High rotor solidity is likely to be a requirement for the complexity of this machine rather than an aerodynamically desirable feature.