update - I realised that I was hopelessly confused on this issue. so I read lots of threads - got even more confused.... decide I'd have to get stuck in & re-test.
so I picked out 4 books that are already in azw3 in my calibre library &
1. I download them to kindle fire tablet, from the library copy that i have in dropbox. the result is as I recalled from previous testing , I get 4 downloaded files that I can't "open with" kindle reader because it won't play. the android OS does not know what to do with the format.
so
2. I delete those, connect via USB instead , send same 4 books from calibre. This time: All OK - they show up in "books" & are readable.
Thus I'm thinking what's the difference, what does calibre do that i did not do manualky..... so I delete those & redo step 1 , leading to step 1.
3. I have them downloaded them but this time, using ES file explorer I move them from my default download folder into the books folder on the kindle Fire. and now they are readable.
Thus the secret , which Amazon don't document for you, seems to be that you have to put the books into the books folder, I guess that is what calibre does.
and in conclusion - you can download your azw3 books from your cloud storage & read them on kindle but only if you ensure they go directly into the books folder, or manually move them there after downloading
please, if I have any of this wrongs, then someone please correct me. I (temporarily anyway) think I've got it now !
( fro the record, I actually used my calibre cloud app to pick the azw3 books out my dropbox calibre library, but I reckon I 'd have got the same outcome by patiently sorting through the folder structure within dropbox then exporting specific files- I suspect that trying to use amazon cloud in place of dropbox would be trickier - put books into a. cloud, let cloud sync them - into documents?., manually move or copy them from documents to books folder.... ? )
it all goes to remind me that my current habit of read in epub format via the mantano app on kindle fire, with calibre companion doing the fetching, is much simpler