Supposedly, John Carter wrote all the happenings on Mars and about the talisman and his body in the tomb so that his nephew can send him back to Mars. But how did John Carter have the time to write the letters and capability to actually send them, knowing he was unconscious on Earth after he got sent back to Earth from Mars? Does this make sense? If so, please let me know. In the meantime, I'll be watching it again.
On their first night, Carter decides to stay forever on Mars and throws away his medallion. Seizing this opportunity, Shang banishes him back to Earth. Carter then embarks in a long quest, looking for clues of the Therns' presence on Earth and hoping to find one of their medallions; after several years he appears to die suddenly and asks for unusual funeral arrangements—consistent with his having found a medallion, since his return to Mars would leave his Earth body in a coma-like state. He makes Burroughs his protector, giving him clues about how to open the tomb. The story reverts to the present, where Burroughs runs back to Carter's tomb and opens it only to find it empty. Shang, in the form of a butler, suddenly appears, having followed Burroughs. But as he prepares to kill Burroughs, Carter appears and kills Shang. He then tells Burroughs that he never found a medallion; instead, he devised a scheme to lure Shang out of hiding. Carter takes his medallion, whispers the code, and is then transported back to Barsoom.
In other words,
Carter was never in the tomb. The whole business with the letters to Burroughs was a setup to lure the Thern assassin out into the open in order to get his hands on the medallion and return to Barsoom.
john carter was zapped back to earth at the end of the movie, upon this he had ten years to search and write..to come up with the idea to set up the final meeting. his journal showed drawings and told of his tale on barsoom, while he was digging as his lawyer said to "ned". then he was off to java and such.