Agree with your first 2 points. :)
But the diesel engine of the locomotive does not show any effect on the ride quality for the diesel-electric locos. The transmission is electric in all mainline ALCO's and EMD's. Stray vibrations etc in the motion of the diesel engine pistons are not directly transmitted to the traction motors as they are not mechanically connected, particularly so in case of the EMD's as they have an AC-AC transmission. But it does result in a less rapid acceleration as there is a limit on the current the on-board generator/alternator can supply.