Did Ellen White changed her beliefs from Father and Son to Trinity?

Today I studied with two friends in church some basics of our Pioneer understanding of the Godhead. I'm writing an additional email for them right now and have realized something odd in the timetable of the official history how the doctrinal development of Ellen White should have happened. You all know the story that she changed her beliefs before writing "Desire of Ages" and published it there.

The first mention of the term "third person of the Godhead" can be found in a letter from February 6 1896: "Evil had been accumulating for centuries, and could only be restrained and resisted by the mighty power of the Holy Spirit, the third person of the Godhead, who would come with no modified energy, but in the fullness of divine power. Another spirit must be met; for the essence of evil was working in all ways, and the submission of man to this satanic captivity was amazing.—Letter 8, 1896, p. 1 (To “My Brethren in America,” February 6, 1896.)"

So if she was a genuine prophet she shouldn't have changed her beliefs after that date back to her former understanding. But in the Youth Instructor of December 16 1897 (almost 2 years later) she wrote: "As the disciples comprehended it, as their perception took hold of God’s divine compassion, they realized that there is a sense in which the sufferings of the Son were the sufferings of the Father. From eternity there was a complete unity between the Father and the Son. They were two, yet little short of being identical; two in individuality, yet one in spirit, and heart, and character." (Youth Instructor, Dec 16, 1897)

Since the article in the Youth Instructor is not a reprint of an earlier article (as far as I know; maybe someone can do some research if this is so), she then must have written it especially for the YI. And that would mean (in the official story) that she must have changed back to her former beliefs. That is simply impossible.

So the term "third person of the Godhead" must have had a non literal meaning to her, and the official story is wrong.