>The scenario that worries me is that years from now, Google and Amazon may be coerced into turning over what I said yesterday
Amazon has said that they're only recording what happens after the hotword is triggered. That's why they can't do custom hotwords - listening for "hey alexa" is built right into the hardware and it doesn't do anything until it hears that. Unless they're flat-out lying, they can't provide any information about what you said yesterday to anybody because they don't have that data.
My 2012 Moto X has a trained keyword (and in fact a dedicated low-power specialized processor to listen for it - one of the things I like about that phone). No "OK Google" for me; it's "Listen please", in Russian. Never tripped by mistake.
The scenario that worries me is that years from now, Google and Amazon may be coerced into turning over what I said yesterday to a corrupt government fishing through the past for word-crime. And boy-howdy, it turns out they recorded everything and retained it forever. Surprise!