Here are a few solutions that can some of the problems to solve..
Surely you will need to test them in practice, before se makes the right decisions as to the shape of the structure of the drive.
Perhaps such a solution with a single tube might be the most efficient, because it will most laminar flow
Or a Cutter Heron
Of course, a whole can be properly enclosed guard areodynamic...
Well, here an example, if the electric drive can also adapt to the helicopter. Turbine itself would be inside the helicopter, but it would not be permanently linked to the rotor, so that no torque moved...
It is air- gear drive
Andrew