1) I wouldn't say community agreed so much as set by PG. But yes, it's a requirement.

2) They exercise their own judgement. The capability becomes available once you pass a deliberately non-public karma threshold that I honestly couldn't remember if I wanted to.

3) There are. I don't believe their identities are public.

4) No. At least not in general. Sharing non-interesting posts is viewed negatively.

5) For anything that exists on its own page or is longer than a paragraph or two, definitely link. I guess you can make a case for inline quoting when you just want a very short excerpt that can only be found on a long webpage.