Rings do rotate in the grooves. Not only are there xray floroscopy that proved that in the 50's, I also proved it by careful assembly and disassembly when I was a 16 year old, hmm that was many years ago. And the arguement about soft Aluminum and hard rings, well there is lots of info about using Al as a bearing, rings rotating aren't going to wear either the iron, steel rings or the Al piston much because they will not be loaded very much when they are doing to rotate thing, if they were loaded to the point of wearing (adhesion, lack of lube or ? / etc) then they would lock and not move.
