Did Prophet Jeremiah's soul pre-exist his conception in human form?
The answer is NO, human soul that existed in the prophetic mind of God during the time of creation cannot pre-exist as it will clearly contradict the definition of soul which is profoundly and intimately united to a body.
365 The unity of soul and body is so profound that one has to consider the soul to be the "form" of the body:234 i.e., it is because of its spiritual soul that the body made of matter becomes a living, human body; spirit and matter, in man, are not two natures united, but rather their union forms a single nature.
But, I will answer Yes also, when the "pre-existence in question is the soul of Jesus & Mary because Adam as progenitor cannot bind the New Adam & Eve as they existed before the creation time begin.
What I'm trying to say is, Jesus & Mary is above Adam and pre-existed even before Adam was formed.
Jesus human but Divine soul pre-exist in the bosom of the Father, while Mary as the chosen Mother of the Logos pre-existed in the Divine Will of the Father as Blessed Archbishop Fulton Sheed said "Mary is God's DREAMED"
and when God dream of Mary it happened in eternity meaning outside of time.
All other creatures after Adam are his descendants and by God's holy decree will inherit the original sin except for a few chosen souls that was predestined by God to be holy & blameless before the foundation of the world. (Ephesians1:3-4)
How can we prove it biblicallly. The Jeremiah1:5 passage is a perfect example.
The word "Before" does not mean a pre-existent created soul but pertains to the pre-ordained mission of Jeremiah.
The question is, is Jeremiah being sanctified in the womb was cleansed of not only of original sin but including all the stain of original sin?. As St.Paul letter to Ephesians implied "holy & blameless" meaning a spotless soul. Because how can God consecrate an offering to Himself when the offering had blemished.
Ephesians 1:4-5 New International Version (NIV)
For he chose us in him before the creation of the world to be holy and blameless in his sight. In love he[a] predestined us for adoption to sonship[b] through Jesus Christ, in accordance with his pleasure and will..
So, Jeremiah sanctified soul is like the immaculate soul of Mary, the only difference is Mary is "preserve" or untouch by original sin while Jeremiah was touch and cleansed from it meaning from all stain of original sin to become worthy of Consecration.
St.Catherine of Sienna had said "everything was pre-ordained towards the Salvation of Man".
Jeremiah was knew by God before he was formed in the womb meaning God knew what kind of "mission" Jeremiah must fulfill and that is to become a Prophet and he was "sanctified" same like St.John the Baptist was "sanctified" to become "holy & blameless" to fulfill his God given mission while still in the womb.