Ulric:
Merlin opposite rotation was done by an extra gear in the reduction 'box - quite simple and sensible, c.f. Allison who built engines with opposite rotation thus adding complexity and doubling the spares inventory as you say. Never clever, but particularly not in wartime.
Griffon, being a different design, rotated the other way for reasons not connected with this point. Later they had contra-props so, as said above, no torque problems.
The sense of the torque reaction depends on the sense of what is being reacted to - in this case the propellors. i.e. it doesn't matter which way the motor turns to achieve that.
There is a case to be made for the gyroscopic effect of the motor's rotating assembly and I assume the Allison designers believed it was important. Having known well a man who was a test pilot on the de Havilland Hornet (Merlins with lh and rh props) I'm sure he never mentioned the gyroscopic effect but he did say how much more controllable it was than the Mosquito (unhanded Merlins) which did swing on take-off.