As far as I can tell Russell never read J. A. Brown's book. Barbour did. But J. A. Brown did not originate the 2520 year count of Gentile Times. An American clergyman was first. Having ideas similar to someone else is not plagiarism. Plagiarism is the use of another's words without attribution.
The immedite source of the 1914 date is E. B. Elliott. Barbour read Elliott's Horae. He said so. Russell never mentions Elliott, and I think he never bothered to read his book. Some of Russell's associates did.
You'll get a better view of the place of J. A. Brown's relationship to Barbour from Schulz and de Vienne: Nelson Barbour: The Millennium's forgotten prophet. It's on lulu.com. ebook is elsewhere too.
Brown was a silver and gold smith by trade, and very involved in prophetic studies and mission work to the Jews. I tried to trace his education, but while I could trace that of his sons, I could not find where he was educated. His books are a bit prolix, but most religious books from that era are. He wrote a short article for an Anglican magazine about 1810 as I recall. It's much easier to read.
Better, more accurate history than you will usually find on this board is found here