We do offer azw3 files for all of our books. https://standardebooks.org/help/how-to-use-our-ebooks#kindle...
I understand that this is their fault and not yours, but maybe it could be interesting for you to offer one of these formats now that the Kindle browser is actually usable?
Do they actually check the file content or just the name?
If the latter, just a .AZW alias to the actual .AZW3 might work ...
(I can't test, don't have a Kindle nowadays)
Once the files are in the Kindle, it would probably work out OK.