Re: If I edit a photo in Photoshop Elements, I can't seem to re-import it back into Lightroom. If I open the relevant folder on my Mac the image is present, but Lightroom just doesn't "see" it at all.

The proper sequence of steps is the following:

1. In Lightroom, right-click on the photo and select Edit In...
2. In the Edit In... dialog box, select the external editor and other appropriate options
3. Edit the photo in the external editor
4. When finished in the external editor, do a SAVE and not SAVE AS

Is this what you are doing? If you are following any other sequence of steps, it most likely is the cause of your problems.