Just yesterday I started using the Kindle Previewer to convert epubs to mobi and sideload them into my "Kindle Content" folder (on my PC). I had been using Calibre to do the conversions, but I found that the mobi files Calibre generated showed only generic thumbnail covers when viewed with Kindle for PC. Kindle Previewer solves this problem. The mobis created by Kindle Previewer show the warnings you mention, but look fine in Kindle for PC.
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