Second, it would only help to validate that a video was taken (and possibly that it was not modified? But I suspect a great deal of video footage gets modified after capture, for light balancing and whatnot.)

Beyond that, however, the problem is larger: videos can be started after a person incites another. They can selectively film parts of an incident, so you miss details off-camera. They miss context, can push an agenda -- all of which are (IMO) the bigger problems than validating a video provenance. (Also, the latter would require skepticism of all non-blockchain'd videos -- something that would take a major cultural shift.)