It's hopeless. Google has already given up on a bunch of end-to-end encrypted projects, and now I've noticed they've added automatic backups of messages to the cloud for Allo, which may include the messages in the "Incognito mode" (I'm not sure, but I wouldn't be surprised if they did that).

I've already moved on to ProtonMail. The days of Google being the "good guys" that will fight for you are over.