I suspect that any editorial changes that happened as part of the journal's acceptance process - unless they materially changed the content - would also have to be kept back as they would be part of the presentation of the paper (protected by copyright) rather than the facts of the research.

No, in practice we update the preprint accordingly.

As an outsider that's a reasonable thing to suppose based on a plain reading of copyright law, but in practice it's not true. Researchers update their preprint based on changes requested by reviewers and editors all the time. It's never an issue.