1. The only way I would know to trick people who do not have access to view things in that scenerio would be to save the photos outside of the protected area and run the program just in the protected area. Otherwise your looking at extensive code modication. I take it you want to throw some queries and build some thumbs on a non member page somewhere. At any rate if files are locked down by this htaccess the actual files as stated would need to be outside as you know.
2. When you upload a multimedia image the thumbnail is not taken from the image. You are given a chance on upload to upload a separate thumb for that multimedia image or it uses the blank default multimedia image thumb holder.