An over square engine and an undersquare engine, both having the same compression ratio, have exactly the same expansion ratio. Bore to stroke ratios have nothing to do with expansion ratios.
V8 motors typically do not have counter rotating balance shafts. V6 motors do.
The harmful vibrations in a crank shaft are not torsional vibrations, they are bending vibrations. Adding more main bearings DOES alleviate this problem.
A flat six is superior to a straight six for vibration, bending, and rocking forces. There are no commercial diesel flat six motors. So your theory about straight sixes being superior to v8s simply because there are more of them available(empirical) doesnt hold water.
As for your water pump discussion, you could also increase RPM instead of increasing shaft size. But this is a pointless discussion because in crankshafts, the controling design criteria is bending, not torsion. And if you think adding pillow blocks doesn’t do any good for shafts loaded in torsion only, then take out half of your bearings in your pump shafts and see what happens.
As I said, all the three shaft sizes have bearings at 5’ intervals. If you try to use a 1” shaft on a 200HP motor with a pump at 300’, you’re going to break the shaft pretty quickly due to torsion work hardening.
As you say, if the bearings become run out and the shaft, even of proper diameter, has run out, then even a shaft of the correct size will fail. I’ve had exactly that sort of failure in a 1.25” shaft under a 125HP motor. It’s a pain in the chops, to be sure.
Sorry for the dupe: in AC motors, it isn’t easy to increase the RPM. The RPM is pretty much set by the line frequency and the number of winding poles. For most all irrigation motors, your max RPM is 1800RPM no-load, 1745RPM (or thereabouts) under full load (accounting for what is called “slip”). You can use variable frequency drives to change the RPM, but it is pretty rare to over-speed these motors, because the turbine pumps are designed for the full-load RPM of the typical three phase induction motor at 60Hz.