

At least (to my knowledge) the Signal messages are decrypted on the client end, so buying the company doesn’t give them automatic access to messages.
Having said that, I’m sure a hostile new owner could update the app to decrypt and then send the messages as plaintext to the servers if they wanted…
Definitely switch to alternatives from Amazon. They treat their authors abhorrently too. I’ve personally been super happy with libro.fm for Audiobooks (essentially Audible, but you can download the audiobooks DRM-free)