Didn’t Ellen White use the writings of other authors?

Yes, and quite more extensively than many Adventists realize. But we must remember several things: First, the laws on plagiarism were not as fully developed back then; it wouldn’t be fair to judge her by today’s much stricter standards—any more than it is to judge some of Paul’s statements about women from the standards of our culture today.

Second, originality has never been the test of inspiration. Even the writers of the “Synoptic Gospels” (Matthew, Mark and Luke) seemed to “borrow” from a common source, known to scholars as “Q.”