Eternity is outside of time, so it cannot "go back," see. Only time goes back. We use time words metaphorically for eternity to give us something of a picture of it. It's like infinity—if you ever just sit and try to think of infinite things, it feels impossible, like your mind will pop. And the reason for that is simply because our mind is not infinite, it cannot contain it. So we need to think of eternity not as "really, really old" or something "way, way back," rather something fundamentally different than time.
Excellent question, and really the heart of it all. I've been thinking about this question a bit again since this thread came up, and have some more thoughts to consider about it. Now if God were let's say, in his original state, not having the roles and relations that he later picks up in time, we basically do not have a Father and we do not have a Son, and I would argue the Spirit functionally plays the role of something like a Mother, and instead of the archetype family unit, we have basically three siblings who are perfectly equal in every sense. So God starts out triplets, basically.
Because to prove the eternal Sonship, we can do this in more than one way. We can simply prove the eternal Fathership of the Father. Because if the Father became Father God at some point, then he was before that simply Brother God. And we had three Brother Gods. But if we can prove the eternal Fathership of the Father, we have automatically, by logical entailment, proved the eternal Sonship of the Son. And of course that wouldn't work for someone like civic who wants to strip all fundamental distinction of what Son even means and make a Father-Son relationship something different than derivative, but we can dismiss that argument as simply making the term Son meaningless.
So we have two starting states for God relationally here:
BROTHER/BROTHER/BROTHER ---> which later at some point in time after creation turn into a familial heirarchy.
OR
FATHER/FUNCTIONAL MOTHER/SON ---> which never changes but displays something fundamental about God's character.
And we say functional here, because although there are many places the Spirit does have a feminine association, there are also places with a masculine association, and since God is Spirit, we can assume gender for him is not biological anyway.
But you make the mistake of using temporal words again in association with eternity. You say "eternity past," and this cannot be a literal phrase, because it would be self-contradicting. It is a metaphorical phrase to indicate to us that eternity was a state that existed logically prior to time. And in that sense it is "older," in that it came logically first, but not temporally first, as that would be an internal contradiction, and just make eternity a time before time.
Now for the question: Does Scripture indicate to us that God had eternal starting relations and heirarchy, versus God later taking those things up after creation? And for that answer, we may not get as direct an answer as we might like, since Scripture is not written like a text book or encyclopedia, but rather organically and naturally formed over time. We may have to make some deductions and inferences in the dataset we are given, and that is natural, and does not mean that a thing stated less clearly is no longer clear.
Yes, I 100% believe Jesus, that God is spirit. Now we should read it more like "God is spirit in quality," rather than "God is one singular spirit being." That is essentially, "God is spiritual. And those who worship him, must worship him spiritually and sincerely."
So, we see right away, that we should not apply the logic and timing of physical beings to God's relations, since God is spirit without the fleshly component. A major error over the years has always been to give God some kind of natural intrinsic physical body.
So what does spirit mean when it comes to familial relations? How are spirit beings related familially differently than natural or physical beings? Do they have the same limitations? Do they have the same locations? Do they utilize the same functions?
Again you use that misleading word, "eternity past." Eternity is not past, eternity is diametrically opposed to past, it's the opposite of past, eternity is transcending time, simply being above all past, above all present, and above all future. Eternity is not somehow "over and done with a long time ago," eternity still exists, because it is a timeless state completely outside of temporal constrictions.
And this is how God, being atemporal, does certain things in salvation, like unifying us with Christ before we even exist, and one of the strongest arguments against Open Theism or Dynamic Omniscience that seeks to limit God's future knowledge, God can put our actual individual sins laid upon Christ many years in the past or future before we have even committed the sins.
If we focus on the atemporal being and function of God, we can come to discover familial relations that we do not necessarily have to logically frame in time, or bind in our thinking to temporal restrictions, lowering God to our status, essentially making him a "bigger" version of us, that had to do things within the lowly constraints of time. God can do things outside of time.
And it is, indeed, outside of time, that Wisdom was formed as God's first companion rejoicing ever at his side in Proverbs, and it is indeed outside of time, that the firstborn of all Creation was in the Father's bosom, the Word being with God. And I take these things to be the Holy Spirit and God the Son respectively.
The argument that this makes them inferior or created, is simply a limitation of the capacity of our minds and understanding, because naturally, there will be things about the Creator that are simply beyond what the creation can even have the capacity to fully conceive in thought, because creations have natural limitations.
So in conclusion familial heirarchal relations outside of time do not denigrate or lower God to created or temporal status. The Father being eternally called a Father, and not adopting the term later at the Incarnation, shows us by logical proof that he was an eternal Father to something, because otherwise Father doesn't mean anything anymore, and thus shows us an Eternal Son.