a key allows some torsional movement, unless the collar has a gib or you have some set screws very, very tight. A tapered collar essentially gives you a press fit or shrink fit, without pressing or shrinking.

I had vibration in my sail drive prop, lots of theories about why, no external drive shaft involved. I finally changed props which made it quite a bit better. Could have been something about the 4 cyl torsional vibrations beating with the blade pressure pulses because the reduction ratio is near integer and it was 4 blades. Never did get to the bottom of it. a 4 cyl diesel definitely has torsional vibration. Some flexibility will change the harmonics of the system and maybe make it better - or worse.

The builder had some big time vibration in a 54' power boat they built with a 800 some off HP MAN diesel. They eventually hired in torsional vibration consultants. I don't thing they ever did resolve it.