This is maybe one of the few relevant use cases of Blockchain technology: Cameras can add video checksums to a public audit log secured e.g. by proof of work or another trust mechanism. If enough parties archive this log it will be very difficult to forge videos without tampering with the actual recording device, because duplicates as well as tampered videos created at a later time would be easy to detect in the data, and the replication as well as proof of work would make it very difficult to forge the entire audit log.
You don't need a blockchain or proof of work for that, timestamped digital signatures are a thing that's being used already, you just need a proper trust infrastructure.
A blockchain with proof of work solves the problem of decentralization and absence of trust; however, for legal matters, having a centralized root of trust is the simpler way to go and requires much less resources.
However, tampering with the actual recording device is a very relevant risk - I struggle to imagine an attacker who has the desire and capability to make some serious crime, and convincingly fake a video as part of it, but would be foiled because they can't figure out a way to upload it properly in the exact manner as a real camera would.
In my understanding 50.1 % attacks are less of an issue here as they would be easy to spot since individual parties still have the old blockchain when the adversary publishes the new one, so by comparing them the forgery could always be reconstructed (if not averted).
As far as I understand this is a problem for Bitcoin because even temporary forging of the chain allows the adversary to double-spend funds, and once they are exchanged for real-world money/services they're gone. For an audit chain this shouldn't be a problem as it's only for logging and there is no monetary value tied to the chain. Also, the chain could be anchored with a traditional trust model and wouldn't need to be completely trustless like Bitcoin.
You cannot, but with the two forks you can see that someone tried to tamper with the signature of a given video as there will be conflicting signatures. That alone can make tampering unattractive for an adversary.
How so? What if all I want to do is cast doubt on a legitimate video? The point of the video manipulation is to manipulate the human response and casting doubt is just as effective. Moreso since the average person is not familiar with the technology & thus defaults to "eh - it's all fake" because there's no way they can distinguish the likely real from likely fake from "too hard to tell".
