Regarding the PDOC tag, there's a
Quality Check plugin for calibre that apparently lets you remove/fix the tag for mobi files (without reconverting mobi to mobi). Maybe you can get inspiration there or contact the author on how it's done.
Quote:
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Fix Mobi/AZW/AZW3 files for a Kindle Fire to insert an ASIN and set the correct cdetype in the internal data to enable sharing and appear in the correct folder
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