Thank you for your question. The pre-eternal existence of the reality of the Prophet (saw) is understood from the traditions concerning the first creations that God created as being the Prophet (saw) and the Imams (as) (see al-Kafi (2) 443 for example). The rational theorization of what this means is derived from trying to understand how a limited world comes about from an unlimited Creator without that world being something completely separate from Him, as that would result in His limitation. To properly get to grasp with these kinds of questions and their theories a background in philosophy and theoretical mysticism is needed, but I hope that goes some way to answer the first part of your question.
As for the second part of your question, the issue of Jesus (as) being born in the material world before the birth of the Prophet (saw) is different from the ontological precedence of the reality of the Prophet (saw) over all of creation. The reality of the Prophet (saw) is not the material manifestation of the Prophet (saw), but it is his ontological reality as the first creation of God and the most unlimited of His creation. It is this reality that Jesus (as) too was a manifestation of.
May you always be successful.