Thanks for all the good advice. My local auto parts store had nylon locking nuts that were just thin enough that the stud on the wiper mechanism could "catch" most of the nylon. And, yes, several minutes of brushing with a stiff bristle brush cleaned up the splines (when I started, I thought the splines were worn out, but it was just accumulated aluminum shavings).

Intelligent of Volvo to make the wiper arm base the sacrificial part; I'd rather not have spent $200 and several hours on the wiper mechanism itself.