For reflowable ebooks
Mobi is basically dead. You can only really know what Amazon is doing by uploading the epub and publishing, then buying it on 2 kinds of Kindle and an app. If all are on same account it's one purchase. The default is KFX with the more similar to epub older azw3/KF8 as a fallback on older Kindles.
The epub3 can work and be accessible, but epub2 is also accessible and works on more ereaders and apps. Many epub3 features are not supported by Amazon.
The Amazon "Kindle Previewer" isn't doing the same conversion as Amazon does on epub uploads. It's only a rough guide and its output can't be published on Amazon. I've abandoned it. The "Send to Kindle" email or web page and download via WiFi to Kindle and app might be more accurate than the Previewer, but isn't the same as the Publishing conversion Amazon does (I don't know why).
They deliver KFX for any ereader since PW1 and all apps and azw3/KF8 format for K3 to PW1. Since they abolished Download and Transfer mostly the K1, K2 and DX family can't get downloads. Only they and DXG are limited to mobi (KF7).
To fully test you need to publish, buy and unpublish after sync with four things:
Kindle App on Android
Kindle App on iOS
Any Kindle newer than a Paperwhite 1 (Gen5?) for KFX.
Any Kindle from K3 (gen3 keyboard ) or later up to Paperwhite 1, with the last FW (some of the older Kindles didn't have azw3/KF8 originally).
Note that are are several fixed layout Kindle formats (at least one is an encapuslated PDF). The app file ending doesn't indicate format. Older PC or Kindle downloads in onld mobi AKA KF7 can have .mobi or .azw (no 3) endings.
I put images each in their own <p> with nothing else, unless they are very small and several same height on one line (then they are in the same tag) or unless they are full screen.