Jewish girls unmarried by age 12 are allowed to choose their own spouses according to Jewish law, so that answers that question. BTW, Gabriel told her that the King was gonna put a baby in her belly, who would one day be King, so she was okay with it.
Human sacrifice is part of Jewish tradtion. When it became unpopular to kill your first-born son, they switched to animal sacrifice, and then to payinh shekels for infractions of the law. That's why most crimes today have a monetary penalty. The Catholic Kings introduced dungeon time/imprisonment, to keep their subjects in line.
Finally, since the whole tale was made up to create a religion that would bridge Jews with Roman Pagans, Josephus based most of his writings on the Astrological origins with 12 signs of the Zodiac, the suns movements congruent to the Winter Solstice on Dec 25th, and Spring Equinox on Easter.