I happened to look over my April 5th posting on Ebook toolsets for mac, and on a lark, followed the links again. Turns out Kindlegen has been updated, and also, Amazon has released a kindle previewer for mac and PC.
Full info HERE.
I'm looking at Calibre's underlying command-line tools, and starting to think that it may be time to revisit how I generate the HTML for my ebooks. I do a lot of work cleaning up after the html generator I'm currently using. I need to poke at Calibre generally and see what parts of the front-end of ebook generation it can do for me without sacrificing the control I've come to want. I'm not investing any time in it right now, as I have no new content to put into ebook format(s), but there may come a time rather soon.