Lets start with vocabulary:
Immaculate Conception:Mary was conceived without the stain of original sin, she therefore, though subject to the ordinary limitations of human nature carried no inherent moral defect. This is the Immaculate Conception. She still had the capacity to sin because like all humans, including her son she possessed free will, however because she was without the stain of sin inherited from Adam and through the grace of God she was virtuous and without sin.
Annunciation:At the Annunciation the Archangel Gabriel appeared to Mary
The angel said to her, “Do not be afraid, Mary, for you have found favor with God. And now, you will conceive in your womb and bear a son, and you will name him Jesus. He will be great, and will be called the Son of the Most High, and the Lord God will give to him the throne of his ancestor David. He will reign over the house of Jacob forever, and of his kingdom there will be no end.” --Luke 1:30This might also be called the Virginal Conception, though the term that is usually used is the Incarnation, because it is assumed that the Annunciation is when God through the Holy Spirit caused Mary to conceive the baby Jesus. In the Eastern Church the feast is celebrated as the Incarnation of Christ. It is for this reason that the Annunciation is celebrated nine months before Christmas.
Nativity:The birth of a child, in this case the birth of the baby Jesus.
Christmas:Literally Christ's Mass from the Old English Cristes mæsse. Often abbreviated Xmas because the Roman letter "X" resembles the Greek letter Χ (chi), an abbreviation for Christ (Χριστός). So see you are not "taking Christ out of Christmas" by using the Xmas form.
Virgin Birth:The concept that Jesus was born while Mary remained a perpetual virgin. This is the subject of some controversy in modern times, not because Catholics have any doubt that Mary was a virgin until her death, but because there is a question of the meaning of virgin. Does virgin mean in this case someone who has abstained from sexual intercourse or does it also mean a someone who has an intact hymen?
We know in modern times it is quite possible for a woman who has never engaged in sexual intercourse to have a broken hymen. Also for Mary's hymen to have been intact after she gave birth to Jesus would have required another miracle beyond the Incarnation. Not that such a miracle is beyond God's power, but did it occur? So far the Church has not taken a definitive stand.
The movie itself (which since it has not yet been released, I have not seen) seems to stake a claim out on some of the controversies, while still remaining true to the actual Gospels. Mary experiences childbirth, which means the movies has gone with the non-miraculous birth (not the same as the non-miraculous conception)position.
Joseph is in his twenties, old enough in biblical times to have older children by another (deceased) wife, the Eastern Church's answer to the brothers of Jesus mentioned in the bible. [Roman Catholics typically use the cousin definition of the word.]
Joseph struggles with his love for Mary and this child who is not his, quite believable, since even saints are not perfect, and in line with what the bible says. After all, it took a visit by an angel to convince Joseph to take Mary for his wife. His intention to quietly put her aside, rather than publicly accuse her of infidelity, shows in some measure his character and feelings for her. Unfaithful women under Mosaic law were subject to stoning.
I'm looking froward to the movie. I'll review it when I've seen it.