Can you make it a contributory database? I wouldn't mind "donating" my browsing history and page downloads to build the index and train the algorithm.

You'd have to find a way to verify reputation to make sure no bad actors could contribute.

How does Internet Archive verify dumps submitted by Archive Team and other groups? This may already be a solved problem.

Not knowing their implementation details, I’m guessing it could be doable without reinventing much. An oracle could dispatch a P2P archive job to a pool of clients randomly assigned tasks, with both the first to archive and the first to validate being recognized by the swarm somehow, with periodic re-archiving and re-verification, rate adjusted by popularity of site and of search keywords.

Yes, I'm planning to do something like this.