So you would prefer a complete answer whether it is right or wrong than living with the realisation that some things are outside our knowledge. You know in the early church they debated the nature of the spirit and soul and whether God gave a soul to each new fetus, and if so at what stage in its development, or whether the soul and spirit were passed down from the parents and slowly formed in the growing fetus. The thing is, we simply do not know enough to give a definitive answer, they didn't then, we do not now. Your insistance on coming up with a definite answer and building you doctrine of creation on it is simply building a house on sand.
The bible talks of God forming our spirit with in us, it talks of God giving us our spirit, but it does not talk of God creating our spirit. Which is a bit of a problem for you when you want to ignore what the bible describes as creation and make up one of your own.
To answer your first question, God could have given a spirit to mankind like he gave his Spirit to the disciples at Pentecost, he could form our spirit anew in each embryo, impart a spirit to each embryo, have his initial gift of spirit pass down through the generations and form in each embryo. On the other hand the terms soul and spirit are actually metaphors (they both come from word that mean breath or wind a movement of air) now soul and spirit could be metaphors for some sort of spiritual material that is part of our make up, or what we call soul and spirit could simply be emergent properties of increasingly complex brains, our story to the paper and ink of our physical nature, the AI to our physical hardware. We simply don't know. But whatever the nature of soul and spirit, it is still be God forming our spirit and soul in us. However until you know the nature of our spirit and soul, you have no basis to think it is ex nihilo creation.