Let’s take a look at some those statements:
"Before the assembled inhabitants of heaven the King declared that none but Christ, the Only Begotten of God, could fully enter into His purposes, and to Him it was committed to run the mighty counsels of His will." PP - 36
Before the entrance of evil, there was peace and joy throughout the universe. All was in perfect harmony with the Creator’s will. Love for God was supreme, love for one another impartial. Christ the Word, the only begotten of God, was one with the eternal Father,—one in nature, in character, and in purpose,—the only being in all the universe that could enter into all the counsels and purposes of God. - Great Controversy, 493.
Ellen White says Christ is the only being. Now, does that mean there are only two beings [Father and the Son] in the Godhead? Not so, becasue she elsewhere clearly says that there are three beings in heaven:
"Here is where the work of the Holy Ghost comes in, after your baptism. You are baptized in the name of the Father, of the Son, and of the Holy Ghost. You are raised up out of the water to live henceforth in newness of life. You are born unto God, and you stand under the sanction and the power of the three holiest beings in heaven, who are able to keep you from falling…. When I feel oppressed, and hardly know how to relate myself toward the work that God has given me to do, I just call upon the three great worthies and say: You know I cannot do this work in my own strength, You must work in me, and by me, and through me, sanctifying my tongue, sanctifying my spirit, sanctifying my words…" Ms 95, 1906 (7 MR 267).
Therefore it should be very clear that that there three beings. But how is Christ the 'only being'? We find the answer to that question in the following statement. Note that Christ was one in "nature" with the eternal Father:
Christ the Word, the only begotten of God, was one with the eternal Father,—one in nature, in character, and in purpose,—the only being in all the universe that could enter into all the counsels and purposes of God. - Great Controversy, 493.
Christ is one in nature with the Father, but what about the nature of the Holy Spirit?
The nature of the Holy Spirit is a mystery. Men cannot explain it, because the Lord has not revealed it to them.”—Acts of the Apostles, 52 (Desire of Ages, 669).
So how is Christ the only being? Genesis 1 makes it clear that the Spirit of God was active in the planning of the work of Creation. Since the Spirit, while having personality, appears to have no bodily existence [for the nature of the Holy Spirit is a mystery]. This makes the Son alone the “being” who is [one in nature with the Father] that could cooperate with God [the Father and the Spirit].
In addition, we are told that the Spirit inspired the pages (PP 33-34) and we should not be surprised if the page devoted to exalting Jesus would not at the same time exalt the Divine Author of the page - "For he [the Holy Spirit] shall not speak of himself"- John 16:13.
It is also possible that "before the entrance of evil", it was Christ that actively entered into all the counsels and purposes of God since He was the one who was revealing the Father to the angels.
Just like the Holy Spirit may not have actively participated in the heavenly counsel before the fall, we see that the Father did not get actively involved after the fall of Adam:
Ellen G. White tells us that “after the transgression of Adam, the Lord spoke no longer directly with man; the human race was given into the hands of Christ, and all communication came through Him to the world. It was Christ who spoke the law on Mount Sinai.”—Fundamentals of Christian Education, pp. 237, 238.
The Holy Spirit has a different manner of appearance and operation than do the other two members of the Godhead; his nature is a mystery. In fact, the Holy Spirit has chosen a non-appearance mode of operation that is almost transparent. Did he exist from eternity? Yes, He was there when the work began in the heavenly courts to carry out the plan of redemption.
Notice the following statement:
This work began in the heavenly courts.. The Godhead was stirred with pity for the race, and the Father, the Son, and the Holy Spirit gave Themselves to the working out of the plan of redemption. In order fully to carry out this plan, it was decided that Christ, the only-begotten Son of God, should give Himself an offering for sin.”—Counsels on Health, 222.
Therefore, the fact the Holy Spirit is not seen as active before sin does not mean he did not exist as a being. This is very similar to the argument against the Sabbath, when people say that there is no mention of Sabbath keeping in Genesis or of Sabbath instructions in the New Testament. There was not much mentioned, because there was no question! The problem that began in heaven was because of Jesus, not the Holy Spirit, so he was not the theme. But his existence is evident.
Add to these quotes we have the following:
"The Holy Spirit has a personality, else He could not bear witness to our spirits and with our spirits that we are the children of God. He must also be a divine person, else He could not search out the secrets which lie hidden in the mind of God. 'For what man knoweth the things of a man, save the spirit of man which is in him? even so the things of God knoweth no man, but the Spirit of God'" [1 Corinthians 2:11] (Evangelism, p. 617).
These words make it clear he must be a divine being himself, else, how could he witness to our spirits, or search out the secrets of God? EGW said that Christ was the only being to enter into the counsels of God, but according to this quote, so can the Holy Spirit, who searches the secrets "hidden in the MIND of God." It’s like the bible. We are told Christ is the one and only Lord (1 Cor. 8:6 Jude 4) but we find complementary statements that say Father is also Lord (Matt 11:25). Therefore we must consider other portions of the inspired word (Spirit of Prophesy) and add them to everything else to get a rightly divided picture so that nothing contradicts.