Regarding 3): Its just adding the function printDownloadlink on an image.php page of your theme to download that image. like printDownloadLink(getUnprotectedFullImageURL(),'Download')
That's all. You can alternatively use the fullimageurl functions for direct download (the full image link has options on the backend). The difference is that hte downloadlist functions will provide statistics on the download number (and of course they don't directly link to hte file which can be anywhere on your webspace technicallly).
The "list" functions are a bit different for other purpose.