There's speculative info about the exth records here in the wiki:
https://wiki.mobileread.com/wiki/MOBI#EXTH_Header
I don't know enough about this to even know what happens. It sounds like DiapDealer knows a bit (enough to know that Calibre overwrites it).
So, I would guess we want to preserve the Amazon exth records. Is it possible to preserve them and have Calibre update the metadata, or is the Calibre metadata stored in the exth record fields?
I don't use Kindle on FB, but, I would like to be able to Whispersync my reading progress on books bought from Amazon and de-DRMed.
Gwen