While we could implement a system to have tamper proof verification of stuff that's been archived, how do you prove that the archive (snapshot) is what was actually at the target at creation time and hasn't been modified before archival?

I'm thinking in purely technical terms here, and that I can easily argue that such evidence is inadmissible, unless there's a certain number of independent, trusted snapshot services that would archive each link too, and allow one to compare the decrypted archive. To achieve that, one would probably want to use an indirection for linking so that such archives do not have to archive everything that exists, but just those that go through said indirection.

Then, the last question is how many independents you need to put trust in the verification, considering that motivated attackers could circumvent, say, the 3 most used archiving services. Or you could bind the archival of a link in a federated number snapshot services to a consensus algorithm. Then you'd ask yourself how many networks (which consist of multiple services) you'd trust.