[floatingatoll]

Your usage of the word 'trivial' appears to be a substitute for the word 'conceivable', and I would absolutely agree that's it conceivable with this specific rewording in place:

> it'd be conceivable for someone to use amazon's firmware keys to sign a new firmware

However, I disagree with the word 'trivial' and the framing it implies. It would require a non-trivial amount of effort for Amazon to do this; both to keep it secret within Amazon; and to build, sign, and ship a custom firmware; and to avoid detection by changes in the device's bandwidth patterns.