How will the sites in question learn of that court order? YouTube is one centralized site, that's different to the internet as a whole.
Sites would be motivated to check it regularly to avoid being out of compliance.
the internet definitely remembers forever, unfortunately, especially porn
The same set of values, yes.
> Why would that be? There can be different algorithms, resolutions, crops...
Yes, so you do normalization and reduction before computing the hashes.
And ‘hashes’ doesn’t have to mean literal SHA. Any non-reversible fingerprinting algorithm will do.