Help to reconcile nozzle diameter, deposited line width, and wall thickness in Cura
So, I think I may have found a satisfactory answer. Cura 3.6 includes separate parameters for line width and shell count:

  

This seems to decouple the line width from the specified nozzle size and target what I believe is a more optimal width (~110% the nozzle diameter).  It was the other version of Cura that was driving a lot of confusion with the line width being defined by the nozzle size.  This also removes the weirdness of not being able slice lines equal to the line width.

Thanks to all who responded.