It depends on what your output is going to be. There is no simple answer to the proper amount of sharpening. Any number of factors can impact your decisions on processing an image. How it will be displayed, what the lighting will be like where it is displayed, what type of medium will be used to display it, what kind of feeling you want the display to evoke, stylistically what you want it to look like... all of these factors and more play in to how you make your decisions.
In general, the closer you can work to your intended output format the better. If you are planning to display on a high density display medium, then editing on a retina display makes a lot of sense. If you are expecting to mostly be viewed on lower density displays, it would make sense to work with those.
If your content will be consumed on many personal devices, my recommendation would be to use a display that has roughly average display density (or simulate it as you suggest by zooming) such that the amount of sharpness is right for the average. For color, I'd go with the most accurate color you can get. Consumer devices won't show the same thing most of the time, but being fixed to standard will center you relative to what the user expects from their display normally.