Lowlevlflier, you got some of it right & some of it not so right.

Multi-blade props are not used to reduce diameter -- they are used to increase blade area, thus permitting the use of more powerfull engines within the physical constraints of the airframe that limit prop diameter.

In your 3,000 hrs of flying I doubt if you have encountered "identical" aircfraft with both 2 & 3-blade props. I very strongly suspect that the 3b varients that you have flown were actually more powerfull than the 2b versions -- that is the common reason why manufacturers end up using multi-blade props -- to transmit the extra power from an uprated engine, where it is no longer practical to increase blade length. Another case is where the 2b versions were not optimally proped to begin with (like the AT-6/Harvard). If simply going to a multi-blade prop was so magical, you wouldn't see 2b props on anything.

You are quite correct about noise/vibration & the perceived effect of higher pulse repetition rates.