Rather than being a plagiarist, Russell was an adapter. Plagiarism implies the use of another’s words without attribution. Russell adapted other’s ideas for his own use. That’s not the same thing. But it is fair to say that Russell never had an original thought in his life.
Russell stuck to the 606 BC date. The 607 revision came later. But both dates have a history. Russell never read John A. Brown’s Even-Tide. Barbour did, but Brown does not point to 536 BC or 1914. He pointed to 1917.
The immediate source of the 606 date was E. B. Elliott’s Horae. The 607 date was advocated by Samuel Davies Baldwin. See his Armageddon: Or the Overthrow of Romanism and Monarchy’ The Existence of the United States Foretold in the Bible, Its Future Greatness; Invasion by Allied Europe; Annihilation of Monarchy; Expansion into the Millennial Republic, and its Dominion over the Whole World, Applegate and Company, Cincinnati, 1863, page 424.
Russell and his associates did not live in a vacuum. All this was discussed widely in the 1870s, not just by millenarians and Adventists, but by main-stream preachers. Russell, contrary to WatchTower claims, was not an independent Bible student. He read what others wrote and kept what appealed to his imagination, scriptural or not.
Some of this history is recounted in Schulz and de Vienne’s Nelson Barbour: The Millennium’s Forgotten Prophet. You should read both their books. Visit their blog here http://truthhistory.blogspot.com/