Hi,
Yes, as Blossom said, if these are commercial copyrighted ebooks, please edit your post and remove the FilesRequested.zip attachment. They should be removed from Mobileread asap.
Both files read / viewed perfectly fine using the very latest Kindle Previewer.
Both mobi ebooks both unpack perfectly with Mobi_Unpack.py and the only unusual thing is the use of # <div> <em> tags in the metadata for Description. I looked at the epubs you sent, and they do not have the extensive metadata Description with its html tags at all. So that was something you added inside of calibre. I am not sure what the use of html tags might do when parsing the information. I do not own a Kindle Fire device to test with.
If removing the Description does not help, then creating a private "issue" in the bug reporting site for Calibre is your best bet. That way the ebooks are protected from prying eyes. Please provide step by step instructions on how to recreate the problem when using a Kindle Fire and let him know that the Kindle Previewer does not seem to have a problem with them.