Techie,
Your right on the mean piston speed not changing, overall the piston speed will average out no matter what rod length. What the longer rod does help is, the piston moves slower from 90 degrees btdc till 90 degrees atdc. This causes less mechnical stress due to lower instananeous velocity of the piston between those crank angles. The piston will accelerate away from TDC much more slowly than a short rod motor. The slower motion translates into a lower instantaneous volicity and hence lower stresses on the piston, piston pin and rod.
And of course the more widely know and understood fact that the longer rod will have a lower angle of the connecting rod with teh bore centerline during teh engine cycle. As you also listed.

Man I have to read and reread the effects the different rod lengths have on an engine many times, and each time understand a little more.
But shoot what can you expect from a chemical plant worker living in Louisiana for 36 years which is all but 3 years of his life.