In regards to Virgin Birth or Virgin of Virginity Birth Occurrence:
1. Does any contradict the law?
What law in particular? The young woman Mary was confused by the Gabriel's announcement of her pregnancy because she had never had relations with a man, but since this was God's doing, she humbly accepted the assignment as a great privilege.
Joseph was having none of that because he knew that pregnancy doesn't happen unless the woman has been unfaithful. The law stated that those guilty of sexual misconduct could be put to death. They were betrothed, but not yet married so he decided to divorce her secretly to preserve her dignity. It took God's intervention with a dream to convince him that this was God's doing and that Mary should be taken in marriage. He did not have relations with Mary until after the birth of Jesus. (Matthew 1:24-25)
2. Did any occur in OT?
Virgin births? None are mentioned....none were necessary.
It was necessary for Jesus to be born as a human child to atone for Adam's sin, but he he could not be born as a sinful son of Adam...he had to be a sinless son of God. Only Jehovah could transfer the life of his son in heaven to the womb of a Jewish virgin to provide the ransom demanded. (Matthew 20:28)
Jesus had both the religious credentials as well as the physical ones to claim to be the Messiah. His life course showed that he had both.....and his miraculous powers proved that he also had God's spirit.
3. Did King Ahaz witness any Virgin Birth or Virgin of Virginity Birth as a sign?
No, the prophecy of Isaiah 7:11 may have had a first fulfillment in the case of the prophet Isaiah himself, through his having a child by a young woman or maiden. The birth of this child named Immanuel would, of course, not be a virgin birth. The reasonableness of this is seen when we consider that the prophecy of Isaiah 7:14 concerning the birth of 'Immanuel' by a maiden was to be a sign for the benefit of King Ahaz to whom the prophet Isaiah was then speaking. The virgin birth of a child seven hundred years later could hardly be a sign to King Ahaz during his lifetime.
Isaiah said under inspiration: “Behold, I and the children whom Jehovah hath given me are for signs and for wonders in Israel from Jehovah of hosts, who dwelleth in Mount Zion.” (Isaiah 8:18, ASV) So this child 'Immanuel' back there, through his peculiar birth and meaning of his name would be a wonderful sign of something to occur future. So Isaiah 7:14 would have a major, complete fulfillment after Isaiah’s day and at God’s own appointed time.
The Christian scriptures tell us that these things were fulfilled with Mary and the son she produced through holy spirit.