Nature journals actually do provide professional copyediting services for all articles accepted for publication. For commentaries (from my own experience), the editing process at Nature is quite intensive.

That just pushes out the argument by one level of indirection. The publishers 'curate and edit' the board of editors for their journals. In turn, these boards of editors curate and edit the articles that go into the journal.

Nope, they don’t. The “volunteer” reviewers are drawn from past published authors, who in turn had their articles selected by previously published reviewers. Turtles all the way down.

This is not true. Reviewers may or may not have been past authors. I have reviewed for many journals in which I have never published.