We should note that plagiarize has a very specific meaning. Holding doctrines similar to or the same as others is not plagiarism. Russell drew from a very narrow set of doctrines, those held by the Age-to-Come/One Faith movement as represented by The Restitution, a religious newspaper. Some claim he was a closet Adventist. He wasn't. None of his doctrine came from Adventism, and all Adventist bodies opposed him.
He read widely from the religious press, often quoting from or mentioning the books and periodicals he read. But he had a narrow doctrinal set, not really meant to please others. If he had plagiarized from others to draw adherents, he would have taught popular doctrine. He did not.
Nothing he taught is original. But why would we expect it to be? He sought the 'old theology,' original New Testament doctrine. He did not seek something novel as did Mary Baker Eddy.
Again, from our book:
Russell saw himself as a kind of cut-and-paste Bible Student, reassembling from scattered sources the Old Theology. Russell said as much in 1889:
We must disclaim any credit even for the finding and rearrangement of
the jewels of truth. “It is the Lord’s doing, and it is marvelous in our eyes.”
The writer wholly disclaims superior ability or qualification for the
reorganization of the truth in its present solidarity. As the time had come for
the bringing together of the scattered thoughts of past centuries in the
marvelous inventions of our day, – so the time had come for the bringing
together of the fragmentary hopes and promises of God’s Word scattered through
Christendom. To deny that the Lord has simply “poured out” this harvest time
blessing of “present truth” in his own due time and in his own way, would be as
wrong as to claim it as of our own invention. …
It came gradually, silently, as comes the morning dawn: the only effort
necessary was to keep awake and face in the right direction. And the greatest aid
in so doing was the effort put forth to awaken others of the “household of
faith” and point them to the light and in turn to urge upon them the necessity
for serving also, if they would overcome the lethargic “spirit of the world,”
and be ready to go in to the marriage of the Lamb.[1]
[1] C. T. Russell: Views from the Watch Tower, Zion’s Watch Tower, April 15, 1899, page 87.