One final quality of life suggestion - especially useful when dealing with a lot of closely packed objects and spinning the camera round:
Would it be possible to mark/highlight the copied/source objects* somehow?
Currently, you can't tell exactly which source objects you are about to move with M. Also if you press O, it's not obvious that the source objects - which you can later move with M - are the newly placed ones rather than the originals.
* The UN-COPY buffer I think you called it - the ones still in the world, not in-hand