You can't downgrade a Kindle, so, at least that should answer part of your questions

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If Windows sees it (as a storage device, and you can properly access it that way), see the Calibre FAQ and post the relevant debug log. It's unlikely you'd be the first to notice something weird on a months-old firmware, but who knows.
EDIT: Oh, I'd completely glossed over the Calibre version. Yeaaaah, that won't do, c.f., @theducks's answer below.