maxtorq, that's one for Dave Jackson and his magic spreadsheet.
In a nutshell you assume the same column of air is being induced by the blades in both rotors at the same time (ie on a single plane). There is a pedantic argument that the lower rotor sees a higher induced flow, but Nick would go to lengths to point out that only works in ideal hover conditions. At cruise the flow outwash contraction will be some way behind the machine before you need to overly worry about this. I would imagine the FBW uses a laser gyro to trim differential collective for precise yaw control. I'll give you a better answer though when i've read the Leishman paper on my desk.
The other feature to notice is the blade profile. The tips have a nice elliptical taper to produce large vortices with reduced local velocities. The inboard section uses an inverse taper so that the retreating portion in the reverse flow circle can be feathered. Again a compromise which sacrifices some hover performance for improved cruise efficiency.
I'm not sure about that rear structure being a reducer gearbox though. It looks more to me like a pusher prop collective pitch mechanism. Driveline failure is generally the result of joints failing. It can normally be contained by designing a surrounding structure which limits the radial movement of the broken driveline. In the worst case that the single LHTEC800 engine was damged, the main rotor and flight controls still allow an autorotation. FBW will be triplex or quadruplex.
This is an experimental machine, so Sikorsky are naturally being gradual about performance expansion. With 17 engine failures under his belt i'm sure Nick, or indeed any TP, would heartily agree that this is a wise philosophy.