It is tough to convince oneself that all attack vectors are being considered. The key idea is that a coercer or briber cannot always monitor their subjects, which leaves a window of opportunity for voters to cast their desired vote and set up fake credentials for their devices. This assumption, however, falls apart when the coercer or briber asks for voters’ devices and corresponding PIN codes during the voting period. I am motivated by the belief that such an attack vector is, in most cases, unrealistic.

Regarding your suggested attack vector, where the briber asks for a video of the screen showing how the number is displayed on the screen, this can be resolved with fake credentials. When creating a fake PIN code, the voter can specify inputs and outputs to the device with which the video can be taken. Fake credentials can further create fake credentials, so it is not possible to distinguish them.