I won't say you haven't made a valid point, you have as far as Exodus goes with an exception I list below, but Luke is a "special case" situation and can't apply. In fact it works to your disfavor in your argument because God controls when a spirit is sent into a person and has, for most people, decided that it will be when breath enters the nostrils. However, that doesn't mean he can't send in a spirit before life occurs.
Since we don't see leaping babies in wombs, we can assume this was a special case baby with a spirit that other babies don't have since no other baby leaps in their mother's wombs. Additionally, the very argument Helen and some other here are making that spirit is separate from breath hurts your argument in Luke in that way also, if they are correct.
As far as Exodus goes, the same argument against it can be made against the Numbers 5 verses I posted. Being that it was a man made law and interpretation of life, which certain religious leaders were allowed to make. The same law gives the right to stone a man to death when he curses his parents. I don't think that was a law dictated directly from God, but rather a man created law derived under the influence of God, but containing some man-made mistakes. Another mistake was that all virgins will bleed on their wedding night. We see that some of these law and interpretation is clearly man-made.
