Courts will use the best evidence available for a particular case, including all kinds of things that obviously can be less than perfectly reliable. Physical photographs have been doctored (or staged) for more than a century, and that doesn't mean that photos as such are inadmissible as evidence; so the fact that the process for taking video wasn't especially tamper-proof won't by itself be sufficient to disqualify it.

If a video is available and doesn't contain a pre-published key then the courts will still want to use it as useful evidence instead of simply ignoring it - sure, the opponent might have a slightly easier time attempting to contest that evidence as possibly doctored (just as they can do now), but courts will still make a judgement for each individual case.