Thanks Chris, really, I have been struggling off and on with the inability to set shortcuts from preferences and services for months if not years. I always thought it was user error of some kind. Especially since it went on between systems.
This post edged me into the solution for a shortcut I had been sort of looking for for ages, fortunately the menu item was not confined to the contextual menu but on the 'services'. "Add to DEVONthink 3" in Finder since you ask! 