> What does this mean in practice? Who are these humans? When can the network get going again? Would a consensus rule change be part of it, and what type of changes would be allowed in that situation?

We have a bunch of examples of this happening in practice. The humans are usually a mix of the developers, parties important to consensus (miners, stakers) and big ecosystem players.

> It sounds hard to manage this type of maintenance breaks in a trustless way.

When solving a problem that violates your core security assumption you are only longer in the world of security definitions. It doesn't really make sense to talk about "trustlessness". If the protocol is busted, you need to find a solution and get enough people on board with that solution that you can upgrade the protocol.

> Once you have established the trust required for checkpointing the entire blockchain regularly, wouldn't it be much easier to checkpoint every block instead and in an instant do away with all the hard problems of blockchain networks?

The checkpoints aren't trusted for safety but instead for availability. Instead you should think of them like alarms that "something has gone horribly horribly wrong, stop everything, don't transact, don't move, don't touch anything, pull the ebrake."

tl;dr Much like the fuse box in your house, my view is that checkpoints should turn safety failures (electrical fires) into availability failures (electricity is shut off).