After the image has been saved, can I still edit text-boxes & shapes in Preview.app?
Solution 1:
No, the edits are saved directly to the image.
Solution 2:
If you convert the image to a PDF, and then edit the PDF, you can edit the additions in Preview.app, even after saving.
To save an image as a PDF, you can use:
File → Export As PDF...
File → Export...
File → Print → PDF → Save As PDF...