(1) Yes. G+ authorship represents real human authors, and it's not only perfectly natural for an author to contribute to multiple sites, it should make that author more credible, as multiple websites "believe in" their expertise and accept their contributions. I also believe that AuthorRank will consider the diversity of authors writing for a given site to be a positive thing.
(2) AuthorRank doesn't really exist yet, according to most experts, FYI. But I don't know if you can find anyone who'd predict that it WON'T make it into the algorithm soon. PageRank is a completely separate factor & algo. The Google ranking algorithm is a combination of many factors, of which PageRank is one, AuthorRank will likely be one eventually, Panda is one, Penguin is one, etc.
(3) I think that's OK, however, keep in mind that if the user is very active on a personal level, this social online activity won't benefit his/her corporate profile. If that person is willing to spend a little time every week in BOTH profiles, sharing, commenting, interacting, etc., then I think all is good. On the other hand, a social media profile that never interacts...just publishes...that looks like spam.
(4) I would leave their profile alone, and encourage them to use it wherever they are going forwards. If they continue to be active, build more trust and authority, etc., that can only benefit you.
(5) They should. NY Times does. It might not be affecting them now, with AuthorRank apparently not yet part of the algorithm, but it is expected to. Keep in mind, giant corporations typically have a very hard time making changes, and for a massive industry leader like CNN or USA Today, they've got such strong backlink profiles that there isn't nearly the pressure on them to take advantage of new and upcoming factors like AuthorRank.