I do not know when the soul is merged with the body, but I do know that the soul is not a 'growing' entity, but is directly from God, and not 'pre-existant.' This is made very clear in the Bible.
Why does the soul live within the body at first? Because that is how God created things to be.
About science -- since it has no way of dealing with the non-material in terms of reproducible experiments, it cannot afford to take the non-material into account. But then you can no more 'blame' it for that than you can blame a mathematician for not expressing his equations in rhymes! It's just two different things. However just because a mathematician does not deal in rhymes does not mean rhymes do not exist, and just because science does not deal in the non-material does not mean the non-material does not exist. It is important to recognize that the limitations of any one field cannot define the existence of any other field.
Our five senses are the way we get information from the material world. Why should they also be responsible for information from a world they are not part of?
However that has never stopped occultists, has it? The deception they have foisted off on people is massive and deadly. But then, they are usually that massively deceived themselves as well. There is NO way for any of them to know if they are talking to/communicating with the soul or spirit of a person who has died or with a deceiving spirit, is there? This is why the Bible prohibits that sort of thing.
Jesus is God. He commands life and death. Therefore it was well within His perogative to raise the dead.