I’m not even sure if the first is truly a copyright issue unless they’re forwarding the newsletters and claiming they wrote them. I’m aware paying recipients can forward my newsletters, but I don’t think many of them do … I’d expect to see it in excessively high clicks on some of the internal links. I make sure my newsletters carry links to other paywalled stuff, forcing readers to the website. If they were reading a forwarded email they would not have access.
I have no “don’t share this email” instructions, and on other sites I pay and subscribe I’ve not seen things like that. One financial site I read tags the footer with “this first appeared in blah blah” which I’ve been meaning to include.
There was a recent post about sending just the ‘teaser’ part of the post as a newsletter, and then forcing the reader to click through to the website to read the full post. I think this is it, though I’ve not tried it. If that solution worked it would answer your first question.