I took a quick look at some of the writings of EJ Waggoner. He seems to not state clearly if he is trinitarian or not. He did say that Jesus was the "comforter" promised in John 14.16; thus diminishing the personhood of the Holy Spirit.
“Jesus is the Comforter. “If any man sin, we have a Comforter with the Father, Jesus Christ the righteous.”(1John 2:1 r.v., margin.)(EJ Waggoner The Everlasting Covenant Page 302)
I have been interested in his teachings the last couple of years. He died in my dad's hometown 13 years before dad was born. So far (even though the spelling of our last name is identical) I have not found if we are actually related.
You have to understand that they were all being shown these truths and especially after Ellen White wrote
"The Pre-existent, Self-existent Son of God.--Christ is the pre-existent, self-existent Son of God.... In speaking of his pre-existence, Christ carries the mind back through dateless ages. He assures us that there never was a time when He was not in close fellowship with the eternal God. He to whose voice the Jews were then listening had been with God as one brought up with Him.--Signs of the Times, Aug. 29, 1900.
He was equal with God, infinite and omnipotent.... He is the eternal, self-existent Son.--Manuscript 101, 1897.
From Everlasting.--While God's Word speaks of the humanity of Christ when upon this earth, it also speaks decidedly regarding His pre-existence. The Word existed as a divine being, even as the eternal Son of God, in union and oneness with His Father. From everlasting He was the Mediator of the covenant, the one in whom all nations of the earth, both Jews and Gentiles, if they accepted Him, were to be blessed. "The Word was with God, and the Word was God." Before men or angels were created, the Word was with God, and was God.--Review and Herald, April 5, 1906.
Christ shows them that, although they might reckon His life to be less than fifty years, yet His divine life could not be reckoned by human computation. The existence of Christ before His incarnation is not measured by figures.--Signs of the Times, May 3, 1899.
Jesus declared, "I am the resurrection, and the life." In Christ is life, original, unborrowed, underived. "He that hath the Son hath life." The divinity of Christ is the believer's assurance of eternal life.--The Desire of Ages, p. 530 (1898)