But it wouldn't be validated with the original key, so people could know it could have been tampered with.

If the key is just the recording device, why not? Play a fake on some other device and record with your own.

Datestamps are part of the keyed metadata.

My camera has no clue what date it is beyond what I tell it the date is.

Which makes video evidence from your camera more suspect than video evidence from a cellphone (which, sure, could be fake-dated as well).

Which further reinforces the fact that there is a tremendous amount of real evidence that will be thrown out.

The idea is that the blockchain tracks the date the video was entered onto the blockchain verifiably. Note that this isn't the same as the date the images were taken, but it confirms that the recording is at least X days old.

How would that relate to the hashing explained here?

But then the metadata, e.g. gps coords, time of the recording, etc. of the recorded video will not match with what is displayed. So it would be an obvious fake.

GPS can be spoofed by replay