Once you compromise the device it doesn't matter how many keys there are because they all have to be on the device.
Or, you'd adapt a hardware wallet to do the same. Either way, there'd be two devices from two manufacturers, communicating over a transparent protocol.
This image can now be injected into the news cycle and be taken seriously.
Keeping a secret on the camera is more or less the "jailbreak problem", aka being able to make a dvd player or console that only does the things that the manufacturer wants.
I will be pretty sad if this problem is actually "solved" effectively because it portends a future where manufacturers can completely curtail user freedom. But...
If the key on the device is compromised by reverse engineering or because the manufacturer is compromised then the whole root of trust is gone.
Attackers can choose the weakest brand of camera.
You now have some doubt about any image that has been "verified" because it could have come from a compromised camera secret.
All the rest of the stuff, the blockchain, etc depends on the root of trust in the camera.
You also now have debates around camera country of origin because there is no near future world where governments can't lean on local manufacturers. Finally, doubt can be thrown on images retro-actively if we later learn that a camera we thought was secure, has vulnerabilities.
All of this would be optional, of course. People can always edit out metadata. But it would lend credence and authority to a photograph or video. Which would carry more weight in a court of law, for example, where this stuff really matters.
I think the disconnect might be that you are working from a threat model where someone else is trying to tamper with an image from your camera. I agree you could make that arbitrarily difficult. But I am primarily concerned with the threat where someone is tampering with their own camera to fake evidence. This possibility makes it harder for you to prove that your own image is legitimate.
Your reasoning implies that one or two hacked cameras are not a problem. To my way of thinking, the possibility that "at least 1 hacked camera exists" or "any camera could possibly be hacked" are enough to throw doubt on any "verified" images that someone tries to present in a high stakes context such as national security or the courtroom. We're back to debating provenance.
It would require a larger degree of premeditation and effort to convincingly pull off a deep fake in that world.
It's not a technicality if the video can't be distinguished from perfect nonsense. Just as you don't let someone with a documented history of hallucinations testify on their own.