Technically, no it doesnt. It stops mattering at the manger.
...and to the divinity of Mary's child.
Can you elaborate on that? Are you saying that Jesus was made divine?
Are the nativity stories an essential part of Christianity?
Absolutely. Explain how and why the Virgin Birth (a single event in time-and-space) demands a Perpetual (never ending) maintenance of Mary's virginity.
See Hermann's #92.
The excelsior that wraps the gift for protection is not the gift itself.
Well, The Virgin Birth of itself doesn't demand anything. After all, Heracles, according to Greek mythology. had such a birth, What demands this is the personal relationship of Mary and Joseph to their God. Yahweh, unlike Zeus, cares for his people. Mary became the spouse of the Holy Spirit, not merely his tool, and therefore Joseph became her guardian and the guardian of her child, who is Yahweh incarnate. It is not required that Joseph be a virgin, but that he be ever afterwards chaste, for to be otherwise is not to fulfill his role. I think that all of this is consistent with the content and tone of the first two Chapters of Luke: Mary and Joseph--and their Golden Child.