Jan, The Merlin being a V12 had a power stroke every 1/6 of a revolution, whereas a four cylinder engine rotates 1/2 a revolution between power strokes, so there was a lot less opportunity for torque reversal with twelve cylinders. I'm not aware of any torque limiter or torsional damper on the Merlin to protect the gearbox, although I'm sure there will be a Ppruner who will know. I suspect the reduction in vibration was one of the reasons for going for so many cylinders, as well as to reduce the cross-sectional area.
The Merlin was also designed with a gearbox from the start, so the loads would have been taken into consideration in all parts at the design stage.
Thanks for the correction about the French autoroute speed limit. I knew it was high than UK motorway speed, but couldn't remember by how much.
As an aside, I've just found this description of the Dieselis, which if the figures are to believed would make anyone paying Lycoming running costs cry.
DslGB001