Calibre is ok but I would not publish a book created in Calibre (It has a forum here by the way). Better to use KindleGen to create the mobi from your ePub. It will create a submittable file to Amazon and will create both the old mobi content and the newer KF8 content in the same file. It can be tested with whatever you wish and it will work the same as what you are doing now.
Dale
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