Can you show me where John the Baptist understood the good news of Christ's birth in Scripture?
John was still in the womb of his mother. He did not physically see anything. Therefore, his "joy" can only be accounted for by understanding something revealed to him while in the womb. The text tells us plainly what it was that John "leaped for Joy" which was heard by him in the womb. It was the announcement:
41 And it came to pass, that, when Elisabeth heard the salutation of Mary, the babe leaped in her womb; and Elisabeth was filled with the Holy Ghost:
42 And she spake out with a loud voice, and said, Blessed art thou among women, and blessed is the fruit of thy womb.
43 And whence is this to me, that the mother of my Lord should come to me?
44 For, lo, as soon as the voice of thy salutation sounded in mine ears, the babe leaped in my womb for joy.
The fact is that a babe still in the womb can experience "joy" and filling of the Holy Spirit. The "joy" was in response to Mary's salutation. Moreover, the filling of Elizabeth was in direct connection with the "joy" of John in the womb and that filling announced the birth of the promised Savior "the mother of my Lord." His joy and filling from the womb is in direct connection to Mary and this prophecy of the birth of the Savior.
Then, can you explain why John sent two disciples to inquire if Jesus were the Christ they awaited?
King James Version (KJV)
1 And it came to pass, when Jesus had made an end of commanding his twelve disciples, he departed thence to teach and to preach in their cities.
2 Now when John had heard in the prison the works of Christ, he sent two of his disciples,
3 And said unto him, Art thou he that should come, or do we look for another?
I have explained this to you twice already but you do not have "ears to hear." John believed in the same gospel all prophets preached - remission of sins by faith in the coming Christ - Acts 10:43. However, when John was in the womb the Christ had not been born, but nevertheless he leaped for "joy" in connection with the birth of the Christ. Up to the coming of Jesus to John for baptism the personal identity of the Christ was not confirmed by heaven. John was told that the Father would confirm the identity of the Christ by the Holy Spirit coming upon him as a dove:
31 And I knew him not: but that he should be made manifest to Israel, therefore am I come baptizing with water.
32 And John bare record, saying, I saw the Spirit descending from heaven like a dove, and it abode upon him.
33 And I knew him not: but he that sent me to baptize with water, the same said unto me, Upon whom thou shalt see the Spirit descending, and remaining on him, the same is he which baptizeth with the Holy Ghost.
34 And I saw, and bare record that this is the Son of God.
However, the personal identity of the Christ was not required to believe in the Christ (Acts 10:43a) or to obtain remission of sins by faith (Acts 10:43b). His personal identity was another aspect of progressive revelation. However, the gospel has from Genesis 3:15 contained sufficient information for the understanding to believe, be justified (sins remitted and imputed righteousness) and for God to use it as His creative word to bring them into spiritual union (regeneration)