Not being a Calvinist, I was not familiar with this dispute among Reformed theologians. I know that the Scriptures proclaim that the fullness of the Deity dwells in Christ, and I certainly proclaim that he is True God, fully 100% God, and that there was no time in which the Son did not exist; however, it is difficult to talk about the Son in exclusion of the Father. In fact, without the Father, he would not be Son. That is in itself a relational term.
I may only be mudding the waters here, and not clarifying anything. This is not a debate with which I am familiar, and so I have more questions than I do answers.
"...However, if each person of the Trinity is fully God, then what of their relations as causal activity? Calvin speaks of the Father as the unbegotten one who begets the Son and the Spirit as proceeding from the Father and the Son. The question then arises whether this is not a violation of Calvin’s strong affirmation of the Son and Spirit’s full divinity. How can Calvin say, for instance, “Therefore, when we speak simply of the Son without regard to the Father, we well and properly declare him to be of himself; and for this reason we call him the sole beginning. But when we mark the relation that he has with the Father, we rightly make the Father the beginning of the Son” (144)? The Son considered in himself as God is said to be unoriginate (indeed, eternal and self-begotten as God has no origin outside himself), but considered in relation to the Father he is begotten. Are we to say then that the Son depends on the Father for his existence? Does the Father cause the Son to be (and so with the Spirit)? Paul Helm, in his book, John Calvin’s Ideas (Oxford 2006), deals with this issue and finds an answer in remembering that the begetting of the Son is an eternal begetting such that the Son is always begotten of the Father. What this eternal begetting means most importantly is that this ordered relation between the Father and Son is what God is and it could not be otherwise. As Helm states: "…the Father’s act of begetting the Son is necessary, not voluntary. It is an essential feature of the Father’s person. And likewise the begottenness of the Son is essential to his person as Son. Father, Son, and Holy Spirit not only are essentially God, the relations to each other that they in fact have are equally essential. But if the Son of God essentially and necessarily has the person he has, then the begetting of the person of the Son by the Father cannot have any voluntariness about it. In a real sense, it was not up to the Father whether one of the divine persons should be the person of Son, since being the person of the Son is essential to the person who is the Son. (56)..."
Calvin - The Institutes Book 1 Chapter 13 section 19:
Moreover, this distinction is so far from interfering with the most perfect unity of God, that the Son may thereby be proved to be one God with the Father, inasmuch as he constitutes one Spirit with him, and that the Spirit is not different from the Father and the Son, inasmuch as he is the Spirit of the Father and the Son. In each hypostasis the whole nature is understood the only difference being that each has his own peculiar subsistence.
The whole Father is in the Son, and the whole Son in the Father, as the Son himself also declares (John 14:10), “I am in the Father, and the Father in me;” nor do ecclesiastical writers admit that the one is separated from the other by any difference of essence. “By those names which denote distinctions” says Augustine “is meant the relation which they mutually bear to each other, not the very substance by which they are one.” In this way, the sentiments of the Fathers, which might sometimes appear to be at variance with each other, are to be reconciled.
At one time they teach that the Father is the beginning of the Son, at another they assert that the Son has both divinity and essence from himself, and therefore is one beginning with the Father. The cause of this discrepancy is well and clearly explained by Augustine, when he says, “Christ, as to himself, is called God, as to the Father he is called Son.” And again, “The Father, as to himself, is called God, as to the Son he is called Father. He who, as to the Son, is called Father, is not Son; and he who, as to himself, is called Father, and he who, as to himself, is called Son, is the same God.”
Therefore, when we speak of the Son simply, without reference to the Father, we truly and properly affirm that he is of himself, and, accordingly, call him the only beginning; but when we denote the relation which he bears to the Father, we correctly make the Father the beginning of the Son.
Augustine’s fifth book on the Trinity is wholly devoted to the explanation of this subject. But it is far safer to rest contented with the relation as taught by him, than get bewildered in vain speculation by subtle prying into a sublime mystery.
—Institutes of the Christian Religion