Sauron, I'll try to give you some answers to your querries:
1. In the BoLT I&II the same stories as in
The Silmarillion appear. The stories in the BoLT are, however, from an earlier dat than the ones in the Silm.
It was the first set-up of Tolkien's work on Middle-earth's history. The structure is also different. In the BoLT, a human named Eriol or Aelfwine arrives after many travels at Tol Eressëa and learns there all the tales about Arda from the Elves living there.
2. Beleriand was actually much more than a land, it was half a continent. It was destroyed at the end of the First Age during the Valar's War of Wrath on Morgoth. Read all about it in the Silm! Here's the map from the Silm (it's big, btw, so I'll just give the link):
http://lotrmaps.middle-earth.us/maps/r3t_M147.jpg
At the right side of the map, you see the Ered Luin. You can see them also on the left in the map in the LotR.
3. I am not familiar with that Atlas, but there is an early schematic map made by Tolkien of the whole of Middle-earth in which appear lands to the South of Harad. Here's that picture:
