Covers of sideloaded .azw3 books from Calibre getting replaced, help needed

[hengyu]

Just to get this out of the way, I far far far prefer .azw3 just because how much prettier the footnotes look as compared to the .mobi. The page turning seem to be faster too, although I can't be sure of that.

The problem though is that my Paperwhite 2016 tries to download a cover from Amazon servers to replace the original one from Calibre. Without using the fix ASIN thing from the Quality Check plugin, the device will fail to download the cover and bewilderingly put a blank thumbnail as the book's cover, replacing the already-correct ones from Calibre.

With the correct ASIN these covers are correct most of the time, but these covers are black and white (which is fine), and tend to be in lower resolution, and sometimes outright wrong. And of course you can't choose which cover you prefer, robbing you of any chance of customisation.

Is there any way to work around this? It seems to me that if Calibre could make the thumbnail file in question read-only during the sideload process it would prevent this from happening altogether, but I don't know how to make that happen.

Would gladly appreciate help and advice!