You have two options:
1. Set PSE as your external editor for iphoto and send the photos from iphoto. Save, not Save As (you may need to adjust the PSE editor preferences>Saving files>on first save to save over existing file) and don't change the name or format and iphoto will remember your latest saved edits and the original. However, you must give up the edits to revert to the original file again. If you want to do this, the trick is that the actual PSE editor application is hidden away in Adobe Photoshop Elements 11>Support files:
Alternatively you can export the images from iphoto, edit the exports in PSE and save them with any name or format changes, then import the edited files back to iphoto as new files. This lets you keep as many different versions of your photo as you want, but they are seen by iphoto as different images.