Hmmm.
Jeremiah 1:5 priusquam te formarem in utero novi te et antequam exires de vulva sanctificavi te prophetam gentibus dedi te
- from the Latin Vulgate as translated by Saint Jerome
(I do so love the latin because if you know Latin it is a good thing to compare your translations with modern ones)
Jeremiah 1:5 ‘Before I formed you in the womb I knew you; before you came to birth I consecrated you; I appointed you as prophet to the nations.’
- from the New Jerusalem Bible
This verse would seem to support a pre-existence, HOWEVER - you must rember that we have mysteries because man was not designed to be able to grasp everything is our earthly incarnation.
Since God exists outside of, and independent of, time itself he is capable of omnipotent knowledge without impinging on free will. He knows that you are pondering a choice, the choice you will make, and the ramifications that choice will have. All of this because time is a concept that was created for our benefit.
This is why Angels who also exist outside of time have perfect knowledge, such that in a single instant was known their fidelidy to and love for the lord.
Our lack of perfect knowledge allows for us the privledge (purchased at a terrible price by Jesus Christ), to fall and fall again - so long as we exist “in time”. After we have died, we too exist outside of time and also have such perfect knowledge as the lord grants us the capacity for.
As to the other teachings, they are in fact in other writings not included in either the Jewish Cannon even after the Septuagint of 60ad, or the Christian cannon. But they can be found scattered throughout the Talmud and other books of Jewish Mysticism. As someone who has a very intimate knowledge of the Mormon faith, I suspect that he has been listening to a Kabbahlist and not a Mormon.
I would suspect that such a person is not driven for a search for the truth of the Lord God, but rather is driven by a desire to find solace in a soothing mysticism that suppports his worldview.
Much like a divorced Catholic who becomes Lutheran so that he does not have to gaze upon our crucified lord with shame and be brought to tears because he is unable to keep even the simplest of commandments.
More frightening is the person who would use a condom with their spouse the night before, and then unashamedly go through the communion line, as though they simply didn’t have to follow the rules they disliked and they could still live in the light of the lord.
It takes only a small breath to extinguish a candle - are we so tied up in ourselves that we fail to show our gratitude because it is convenient or because we are “modern”.
If modernity means that we have decided that we are in a better position to decide what is best four our immortal souls then please keep it - that is Satan (not God) who teaches one to rationalize - just as he taught Eve in Genesis. But I have digressed.
Maybe, Chist himself should answer these questions - if there is an adoration chapel in the area then you may go to be with him, and receive your answers and the solace you seek 24x7. I like to start with a thank you, and go even when I have need of nothing - to express to the lord my gratitude for his sacrifice, and hope that my feeble attempts to comfort him may be pleasing to him, much like a small child trying diligently to assist a hurt adult.