

Yes, a creator can create a closed source copy (independent of the GPL’d work), assuming they have full ownership or permission from all contributors
It’s not fully academic only because it’s common for companies to develop a GPL version and offer commercial licenses where the corporate customer is exempt
But none of this affect the users of the GPL version, so most people don’t care


Correct - the notification API from the server is literally just a ping to inform it there’s something to fetch. The app itself fills the notification content. If you tell it to leave it blank there’s nothing cached outside the application storage.
Apps *can* let the server fill the entire notification content without waking the app, but that’s not how Signal works