While the Mormon neo-gnostic conception of the pre-existence of the soul (or person) is heretical and absolute humbug, one cannot disprove it by contrasting the created human soul with the pre-eternal Word’s pre-existence of all creation. Many pious Christians reading the same Scriptures as the rest of us believed the (also erroneous) traductionist theory of the origin of the soul — that all human souls are ‘fragments’ of Adam’s soul, and thus, while not pre-existing the creation as Christ does, pre-existed our conception and birth. Indeed Blessed Augustine’s overly gloomy view of the Fall, termed “Original Sin” in contrast to the Orthodox notion of Ancestral Sin, was originally argued on the basis of traductionism.
Strangely the Latin Church and most protestants have embraced Augustine’s description of the Fall while rejecting his views on the origin of the soul, in favor of the Orthodox position that each soul is created anew at the moment of conception. It is, of course, this the creationist theory of the origin of the soul (yes that’s the correct term, predating the use of the same word vis-a-vis the origin of the universe by a good millenium plus) that you are arguing for, but the traductionists also had their proof-texts (esp. the one about Melchizedek receiving tithes from the patriarchs, and even including the one from Jeremiah you quoted), so the truth of the creationist view is seen in the tradition of the Church (both East and West) as the sound interpreter of the Scriptures, not in the Scriptures themselves.
Thank you for an analysis that was actually enjoyable to read.