Where we may need to tease out the semantics is over when this occured since I hold that this occured in eternity, which is timeless, rather than specifically after the resurrection. It's akin to the question "when did he become the Son?", to which the answer usually comes "was the Father ever not the Father and, if not, how could he be so without a Son?"
I have commonly heard this argument from Trinitarians and it strikes me as contradictory. The verse says, when translated literally, that God "happily chose" this for Jesus. Now to choose something is an action. An action has to be performed for it to take place, and yet to say that it "occured in eternity" is to really say that it was always this way and so it never really happened as the text says, it just was. But as the text says it happened, how can you accept that it was always so?
Now, on the matter of him being Father without a Son, I believe that the same would be said of him as creator. Since there was not always creation, he has not always been the creator. I suspect the answer to this is found in that he has always had the ability to do it.
An additional significant issue is understanding what is mean by "equality" in the Trinity. Most would hold this to be due to their substance being identical ( a point I know you agree with ). Just as you and I are made of the same substance, we are also equal. That has no bearing on what roles we have in life, wherein one might be another's boss.
I would agree with that, to an extent, but also being made of the same substance, a father and a son are made of the same substance, but the father always comes before the son and while they are made of the same type of stuff, they are not a single being as Trinitarians argue for God. I see the Trinitarian position flatly contradicted in the assertion of them being one being in Hebrews 1:3 though, for Jesus is said to be the copy of God's being, or the copy of his substance. Whenever you have a copy of something you always have two, the original (God the Father) and the copy (Jesus Christ).
The reason I conjoin these subjects is because I see the very essense of the substance of the Divine as having an eternal quality. By their very "Divine nature" they dwell in eternity, and hence are not subject to the constraints of time such as ourselves. Hence they are first and last, and have no need to foresee or tweak events, as they exist in an ever-present "now", or "I AM".
I would agree that the substance is eternal. I would disagree that the life is eternal. In other words, Jesus Christ did not always exist, but I believe God formed him of his own substance, of his own being. Once formed, God gave him life. Hence, according to the most ancient punctuation of John 1:4, we read: "What came to be in him was life and the life was the light of men." I believe that after God formed him he then gave him life, and that life came to be in him, much like in the creation account of Adam. We are told that "the life was the light of men" for in the life coming into this one, he created the one that would come to be the Messiah, who is identified as such.