If my history is correct, the Indianapolis had no escort ships. My understanding is that the mission was so top secret that it was 48 hours before the ship was even reported missing. However, If your survivor friend has told you otherwise I will take his first hand account as the truth.
We had no more atomic bombs for immediate use. If historians report that another would have been ready for use on August 21st thats probably accurate. But Truman and top government and millitary officials, after several meetings, believed that the second bomb would have a larger psychological impact than the first one and they deduced that Japan would be too shocked to call our bluff. They were right. When we told them we might have more they were in no mood to find out if we were telling the truth.
The first Jaws movie has a great scene where Brody, Quint, and Hooper are on Quints vessel and sharing after dinner drinks/stories. Hooper and Quint soon begin a game of one upmanship that goes about six or eight rounds. Finally Quint announces that he survived the Indianapolis. The look in Hooper's eyes is priceless. Not only does he know he can't possibly top that, but as a lifelong marine biologist he is in awe just to be in the presence of an Indianapolis survivor. Quint tells the story in ever more apocalyptic tones and it makes for a great moment in a great movie.