Amazon support are talking nonsense.
Covers dont show up on Kindles because of official amazon policy to discourage side-loading of books. See
https://manual.calibre-ebook.com/faq...droid-ipad-etc
And the Kindle indexer getting stuck is because amazon designed it so badly that it crashes when it comes across some books, and there is no way to get any diagnostics from it:
https://manual.calibre-ebook.com/faq...id-not-show-up