[DiapDealer]

You can get a mobi to open to a page that has the cover image in it. But as you've discovered, that doesn't result in a full screen representation of the image (the image that's part of the metadata) on a Kindle.

You simply can't make a mobi that will open to the full screen (metadata embedded) cover image on a Kindle (the older ones anyway... that might change for the newer devices). That's a limitation of the reader software on the Kindle, not the mobi format itself. So nothing calibre could do can "fix" it.