Hi Rex
Re the drivers cupola, I was thinking in terms of having 2 sets of turret rotation gear, 1st one mounted normally under the turret, the 2nd one mounted within the turret & connected to the 1st via the split ring, but with reversed polarity, so that every input to the main rotation gear results in a reversed input to the drivers cupola - ie contra-rotation. Of course whether that would actually work consistently, or would need constant recalibrating to zero it, you'd find out in the wash. A further difficulty, when the tank goes into reverse, the driver's cupola faces to the rear so he can see where he's going. Not sure how you'd arrange that unless there was a linkage to the reverse some how, which is getting way beyond my skills.
It occurred to me that the drivers controls would have to be 'fly-by-wire' as per modern jet fighters, as purely mechanical linkages couldn't be used. Given this was before fly-by-wire was commonly available, it no doubt helped to push the costs up.

Mal