While I am no supporter for Rutherford and I think he did violate Russell's Will by starting the Golden Age I don't think Russell meant to forbid the publishing of new material after his death. Why else have an editorial committee if all they could do was review Russell's old sermons? I think the word "publications" is parallel to "periodical" in that phrase from the Will.
I'm sure Russell thought he was the "faithful and wise servant" and only envisioned the new material to be in harmony with his views. The Memorial issue of the Tower for Russell's death (December 1, 1916) mentions Russell's last words. Among them is the statement about the 7th volume: "Some one else will have to write it." (Now I don't think he would have been proud of Woodworth & Fisher's production, however.)
According to Russell's Will any new material had to be reviewed and accepted by a majority of the editorial committee. It would only be a step further to string together a series of articles into a book. And curiously, almost all the Bible Student groups have done just that. P.S.L. Johnson actually outwrote Rutherford. His Epiphany Series of Studies in the Scriptures goes on and on!