With your External Images plugin - can images be added and updated via the Woo Commerce REST API?
Yes they can. To add them, set the post meta field external_image_url – inside here, place pipe | separated image URLs.
If you are updating products, if you have your API configured to run the post_meta actions by default, then the same as above will work, just update the external_image_url meta value.
If you have wordpress hooks switched off for some reason, you’ll also need to reset the meta field ei_converted to 0 instead of 1.