@whereu
Following is what the Bible tells or shows me Whereu, in more ways than one, but I'll just cover what you have or that area.
15 The Son is the image of the invisible God, the firstborn over all creation.
16 For in him all things were created: things in heaven and on earth, visible and invisible, whether thrones or powers or rulers or authorities; all things have been created through him and for him.
17
He is before all things, and in him all things hold together.
The Son is the image of the invisible God, -> as was the 1st Adam man, so was the 2nd Adam Man.
Firstborn of creation = we know that is not literal, as he only ever come into existence from Mary, the plan within God, known as the firstborn of creation, right where ALL our creations, Naturally begin, before expressing them out of us, in some form or one way or another.
Same context in the mind of God Rev 3:14 - the beginning of the creation of God, was the Jesus to be, also pre-slain in this plan Rev 13:8.
For in him -> for this reason, for the firstborn one to be...........all things were created.
Through him = lets let the Greeks define this, after the following.
Isa 44:24 -
Thus says Yehowah, your Redeemer, And He who formed you from the womb: “I Yehowah, who makes all things, Who stretches out the heavens all alone, Who spreads abroad the earth by Myself;
Psa 33:6, 9 confirms.
Why did he create;
dia: through, on account of, because of
Original Word:
διά
Part of Speech:
Preposition
Transliteration:
dia
Phonetic Spelling:
(dee-ah')
Definition:
through, on account of, because of
Usage:
(a) gen: through, throughout, by the instrumentality of, (b) acc:
through, on account of, by reason of, for the sake of, because of.
the future one to be, born from Mary.
It becomes very clear from the likes of the Isa 44:24 and the overall Greek definitions, as to how we need to view the "through" word used and in fact, the likes of the Isa quote and more, Demands this, whether we like it or not.
Before all things -> as in the firstborn one to be, in the mind of Yehowah, everything was going to center around and for this one and for this reason -> "in him (for this reason, above) all things hold together".
Now a KEY verse you left off;
1:18 - And he is the head of the body, the church: who is the beginning, the firstborn from the dead; so that in all things he might have the preeminence.
And now after the cross Heb 1:3................and upholding all things by the word of his power,......................the words that the God and Father raised him in and gave him.
Had he failed the works that the Father gave him (not likely), he would not have been raised and not holding preeminence.
Not to mention, a pre-existing one would be already holding this position, or at least jointly with the Father.
However, Jesus received all things that he has, from the Father, the one Jesus defined as the only true God, independently of himself, when he was here, in John 17:3
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PS -> "Did Paul write about a pre-existent Jesus?
"
That's a fair and reasonable approach, raising a point with a ? mark. 
