I cant help you with your first question but as for the second one I can. It all has to do with bass splits really. If you do a bass split in the down and back up order (1,2,3,4,5,4,3,2) thats 8 counts of playing, it basically comes down to writing music thats even. I think the reasons Cavies march 6 is just for the extra dimension of sound, yes it does add some to the line but its not a huge difference from a 5 bass line and a lot of the times theyll have the top bass play basically the snare part to give it some extra depth.