I think that it should be sufficient to create content identifiers of all unitary parts of the video, e.g. parts between keyframes, and skip over the ones which are not supposed to be there.

These identifiers could be collected automatically by plugins like SponsorBlock in a community effort and then combined together to identify parts which are common for every viewer, i.e. the ones representing the original video content.

In other words, it seems to me that even putting ads directly into a video stream would not prevent people from being able to block these ads.

What are the ad free countries?