In reply to your first question, I must point out that the contemporary SDA doctrine of the Trinity does indeed teach that the Father, the Son, and the Holy Spirit are Three Persons distinct one from the other; but so does the Roman Catholic dogma.  Under the heading “Dogma of the Trinity” the Catholic Encyclopedia Online states:

The Trinity is the term employed to signify the central doctrine of the Christian religion -- the truth that in the unity of the Godhead there are Three Persons, the Father, the Son, and the Holy Spirit, these Three Persons being truly distinct one from another. . . Thus, in the words of the Athanasian Creed: "the Father is God, the Son is God, and the Holy Spirit is God, and yet there are not three Gods but one God.". . . the Persons are co-eternal and co-equal.  (Emphasis added.)

The key to detecting the false doctrine is in the word “unity” in the above statement.  The meaning is defined in the “Catechism of the Catholic Church, Second Edition” as follows:

252 The Church uses (I) the term "substance" (rendered also at times by "essence" or "nature") to designate the divine being in its unity, (II) the term "person" or "hypostasis" to designate the Father, Son and Holy Spirit in the real distinction among them, and (III) the term "relation" to designate the fact that their distinction lies in the relationship of each to the others. . .

253 The Trinity is One. We do not confess three Gods, but one God in three persons, the "consubstantial Trinity".83 The divine persons do not share the one divinity among themselves but each of them is God whole and entire: "The Father is that which the Son is, the Son that which the Father is, the Father and the Son that which the Holy Spirit is, i.e. by nature one God."

254 The divine persons are really distinct from one another. "God is one but not solitary."86 "Father", "Son", "Holy Spirit" are not simply names designating modalities of the divine being, for they are really distinct from one another: "He is not the Father who is the Son, nor is the Son he who is the Father, nor is the Holy Spirit he who is the Father or the Son."87 They are distinct from one another in their relations of origin: "It is the Father who generates, the Son who is begotten, and the Holy Spirit who proceeds."88 The divine Unity is Triune.    (Emphasis added.)

Now I understand that the central point made above is that there is but One God, and He is One Being in three distinct persons.  The Wikipedia Encyclopedia supports this conclusion:

Trinity is the doctrine that God is one being who exists, simultaneously and eternally, as a mutual indwelling of three persons (not to be confused by "person"): the Father, the Son (incarnate as Jesus of Nazareth), and the Holy Spirit. Since the 4th century, in both Eastern Christianity and Western Christianity, this doctrine has been stated as "three persons in one God," all three of whom, as distinct and co-eternal persons, are of one indivisible Divine essence, a simple being.  (Hyperlinks left active in case you want to follow up; emphasis added).)

This understanding is buttressed by a Protestant source.  The online Christian Apologetics & Research Ministry defines the Trinity as follows:

God is a trinity of persons: the Father, the Son, and the Holy Spirit. The Father is not the same person as the Son; the Son is not the same person as the Holy Spirit; and the Holy Spirit is not the same person as Father.  They are not three gods and not three beings.  They are three distinct persons; yet, they are all the one God. . . They are coeternal, coequal, and copowerful.    (Emphasis added.)

That the last quotation is from a Protestant source is significant.  As much as the contemporary SDA Church wants to associate the statements of Ellen G. White with its adoption of the Trinitarian doctrine, it is based on the Roman Catholic/Protestant concept.  Furthermore, here is the “Fundamental Beliefs” Statement No. 2, voted at the 1980 General Conference Session:

2. Trinity:
There is one God: Father, Son, and Holy Spirit, a unity of three co-eternal Persons. . . (Emphasis added.) 

The evidence is irrefutable.  Rome declares, “. . . in the unity of the Godhead there are Three Persons, the Father, the Son, and the Holy Spirit . . . there are not three Gods but one God.". . . the Persons are co-eternal and co-equal;Protestantism echoes, “God is a trinity of persons: the Father, the Son, and the Holy Spirit. . . They are coeternal, coequal, and copowerful;contemporary Seventh-day Adventism re-echoes, “There is one God: Father, Son, and Holy Spirit, a unity of three co-eternal Persons. . .”  It is all of one source.  The Church has drunk from the chalice containing the wine of Roman Babylon.  If all of its other contemporary doctrines were biblical Truth, this would still be the deadly potion that it is.  The harlotry is aggravated by the fact that the Church connects the Writings of Ellen G. White with this false doctrine; but in describing the Godhead she neither used the word “Trinity” nor the terminology of Trinitarianism.  In all of the Writings on the Ellen G. White Estate website, there are only two references to the “Trinity,” and neither of them is the language of Sister White.  One is an editorial introduction to the compilation of statements that are interpreted as Trinitarian.  The other is a paragraph heading in the book Evangelism, which is clearly a gloss on the text.  In the book A Prophet Among You, it is explained that after the compilation of statements from the Writings, “Brief paragraph headings were then chosen.”  The paragraph heading reflected the theology of the compilers only.  The “Trinity” and Trinitarian terminology were just not in the vocabulary used by Ellen G. White.

The Trinitarian doctrine that there is One God denies the biblical proof that there were Two Gods prior to the Incarnation – a duality, and Three Gods after the Incarnation, in Ellen G. White’s terminology - “the Heavenly Trio.”  The contention that Three Persons of the One God are co-eternal and co-equal negates the true humanity of the God-man.  The “I AM” in Jesus Christ is indeed co-eternal and co-equal with God the Father; but the human Jesus is not.  This is why He could make the following statements concerning Himself:

I can of mine own self do nothing . . . (John 5:30.)
My Father is greater than I . . . (John 14:28.)
And this is life eternal, that they might know Thee the only true God, and Jesus Christ whom thou hast sent (John 17:3.)

The Apostle Paul said of the One who was in the form of God, that He “emptied Himself” (Gr. Phil. 2:7,) to take the form of man.  The doctrine of the Trinity thrusts a dagger through the heart of the “gospel of God . . . Concerning his Son Jesus Christ our Lord, which was made of the seed of David according to the flesh” (Romans 1:1.)  Jesus Christ in His humanity was, after His ascension, highly exalted by God the Father (Phil. 2:9.)  It is because God the Word took part of flesh and blood that we who are “partakers of flesh and blood” (Heb. 2:14,) can now become “partakers of the divine nature” (2 Peter 1:4.)  It is because God condescended to become truly man in the flesh that by faith in Jesus Christ the Father has “raised us up together, and made us sit together in heavenly places in Christ Jesus” (Eph. 2:6, 8.)