[Quoth]

Calibre saves plain old mobi (KF7) as mobi
Calibre saves dual old mobi + azw3 (KF7 + KF8) as mobi
I think Calibre saves KF8 (newer Mobi) as azw3

Plain azw files rather than azw3 are usually old mobi with extra DRM. Occasionally I've had azw files from Amazon that had to be renamed mobi to work on anything other than a kindle. Both sorts of Mobi, azw, KF8 /azw3 optionally have encryption. KFX from Amazon always has encryption even if the Publisher asked for DRM free. Avoid KFX by selecting Download to PC and then USB (sideload) content. Might not work for Apps, Fire (really Android with App) or Kindle programs.

The only use at all for dual mobi is to upload for Kindle to Smashwords, because they don't know what model of Kindle someone has.
I think Kindles need at least firmware 3.x.x do do azw3 / kF8 or the newer part of a dual mobi. Older Kindles only do KF7. With a Dual Mobi you may need to select Publisher Font on Font page or else you get the old KF7.

KF7 doesn't have embedded fonts, or at least the Kindle will ignore them. KF7 might only do page breaks at start of files. It may even predate the Kindle.
KF8 uses later HTML and optionally embedded fonts and is much like epub2.

Amazon will convert an epub2 upload to KDP to at least KF7, KF8 and KFX and deliver according to what Kindle eink, fire or app you select if you have more than one kind of kindle.