Dave,
I think this is an example of an idea being of its time...
To work the helicopter needs more than just enough power to enter hover in ground effect. It needs a surplus of power for control, since there are no ailerons to suck manouvreing power from flying speed. It also needs to have an element of stability (often supplied by the pilot) but this is as much for confidence in the machine as for performance. Finally the aerodynamics for helicopters are just much more complicated than for seized wing aircraft.
Like i say, whatever the exact calculations Cornu came close enough to warrant the credit for the first flyable helicopter - even if this was only a gusty hop. From the power point of of view he was only seperated by 1 year from achieving sustained hover.
It would be a shame to think that he was discouraged from further development, but like i say helicopters are complicated. He may just have realised that there were many more issues than just having hover power on tap from the engine. It's quite likely that he continued to think about future developments from what he learned - it would be interesting to find out what he documented (including the patent on continously variable swashplate) although his home was destroyed in 1944 so little probably survived.
Paul Cornu - Wikipedia, the free encyclopedia