Is it possible to run in an emulator, with a fake camera input?

Does this app somehow interface directly with the hardware? Last I looked iOS saved photos to files that apps had to them open up?

Why couldn’t someone else make their own signing system, though, and apply it to arbitrary images?

The timestamp is based on the device. However, some of the security data comes from the Witness validation server and others from the platform (Google Play Integrity/Apple App Attest). Altering the date/time on the device would conflict with that information, and possibly fail the device validation. One of the key criteria for using the app is it will only run on a device that passes validation from the platform. So rooted devices can't run the app, as they can't pass validation.

We haven't started that phase of development yet but the original image would still be immutable. Subsequent edits to the image metadata would be would be done via Photoshop with a plugin for tracking those edits and then saving and certifying that the original image was still locked with the added visual changes. Haven't decided how those additional changes would be tracked, but I'm guessing it's necessary to do so.