I don't know a ton about copyright, but the life of the author + 70 years rule only applies to works first published after 1978, so it would not apply to the NWT. Also, re-printing a work with a new copyright date does not restart the copyright period. First publication is the key date, and last printing doesn't matter, contrary to what was indicated above.
This said, and subject to correction if someone else is more expert on copyright law, I believe that because the original copyright was renewed, the NWT's U.S. copyright period would last 95 years, so until at least 2045. (I believe the "Greek Scriptures" was published in 1950. I'm not sure whether the copyright for the entire publication would expire then, given that the "Hebrew Scriptures" was not published until later). The anonymous/known author distinction that ties the expiration to the death of the author only applies to works first published in 1978 or later. Here is a good summary of copyright term length for various works: http://copyright.cornell.edu/resources/publicdomain.cfm