iPhoto keeps importing duplicates
After waiting for quite a bit of time, I have to answer my own question and do so "negatively": doing what I want to do just seems impossible for the time being. Working with pictures one allows iPhoto to "duplicate on import" works fine, it just means that iPhoto will more than double the storage requirements for the photo library, essentially duplicating it and adding its own thumbnails, Places/Faces metadata, etc. How disappointing. :(