When did our spirits come into being? Do our spirits have a pre-earth life existence? Jeremiah 1:5 (among other scriptures) seems to indicate such.
No, we do not have a pre-earth existence.
Second Council of Constantinople:
If anyone asserts the fabulous pre-existence of souls, and shall assert the monstrous restoration which follows from it: let him be anathema.
Jeremiah 1:5 does not refer to Jeremiah existing before his conception. It simply means that God, who knows everything, knew that Jeremiah was going to exist and had a plan for him.