Hi Joanne - You have some great questions. Here are the short answers to your questions. Perhaps, later we can go into more detail. 

1) Where does the Book of Mormon fit into Bible Chronology? 

The Book of Mormon narration begins in the first year of King Zedekiah, who was the last King of Judah before Jerusalem was captured, the temple was destroyed, and most of the inhabitants were carried into Babylon. (2 Kings 24:18) The prophet Lehi was also a contemporary of the prophet Jeremiah, who was cast into prison by the unbelieving Jews and carried into Babylon with them. So, the Book of Mormon continues from this point in Old Testament history (roughly 600 BC) through the time of Christ and on to approximately 400 AD). One exception is the book of Ether, which is a translation of an ancient record found by the people of Nephi. It relates the migration of the Jaredites from the confounding of the languages at the Tower of Babel (Genesis 11:1-9) and continues until it overlaps the history of the Nephites.

2) Why did the Lord share the great vision about the birth of Jesus Christ with Nephi and not any of the Old Testament Prophets (not in the language used in Nephi at least)?

Good question. First, I think it is a mistake to say that the Old Testament Prophets did not have similar experiences to Nephi's. Many of the prophets had great revelations. We know, for example, that Moses spoke with God face to face. Isaiah also saw the Lord and prophesied of his birth, life and sacrifice in beautiful poetic language.  John, the apostle, saw the same vision that Nephi saw, but only John was allowed to write the end of the vision where Nephi was asked to stop at a certain point. (See 1 Nephi 14:19-28) I think the apparent difference is not in the quality of the experiences, but more of a difference in the way that these prophets shared their experiences. Because of the wickedness of the Children of Israel, many of the Old Testament prophets veiled their teachings using literary devices and symbolic language rather than plain speech. One reason for this is explained in 2 Nephi 28:30 where Nephi says that the more righteous and teachable a people are, the more knowledge can be revealed to them. The Nephites, on the other hand, were more receptive. So they received a more detailed account of Nephi’s vision. Nephi also made it a point to use very clear, understandable language. Nephi explains the reason for this and further explains the mode of teaching that was used by the Old Testament prophets in 2 Nephi 25:1-7.