Yes they were. They were engaged at the time Mary became pregnant. They married right after the angel told Joseph that Mary was pregnant, however they did not consumate the marriage until after Jesus was born.
No. The details of their marriage are unknown, but both Mary and Joseph were Jews, in the line of David, and would not have married in a 'church building'. Though there was often celebration at that...
The Hebrew word Almah refers to a young girl that reaches puberty but still under the care of her parents. Mary being an Almah may have been espoused to Joseph as early as when she was 12 years old....