View and edit/delete photos with Mac Photos App without importing I would like to view my photos and edit or delete the ones that I don't like. I would also like to do this while keeping the photos in their original location (e.g. an external SD card).

Is it possible to use Mac Photos App to do the edit/delete without importing the photos?
As long as the SD card can support the size, create the Photos Library in the location of the SD card (formatted as Mac OS extended (Journaled). You may also have to drag the photos from the SD card into the newly created library from the Finder rather than use the normal import interface sidebar from within the Photos application.