Given how remote Saint Helena is, how is it that nobody attempted to rescue Napoleon either out of a desire for money or allegiance? Couldn't run of the mill traders just have docked and taken him?
Thomas Cochrane apparently had a plan to rescue Napoleon from St Helena and insert him as an emperor in South America in order to counter the growing power of the USA.
How viable this was, given that Cochrane was involved with the various independence movements throughout South America going through the process of removing Spanish control, is another matter.
He got as far as sending one of his ships to St Helena to effect the rescue when Napoleon died.
Sources: Autobiography of a Seaman, Thomas Cochrane Dundonald
The Sea Wolf: The Life of Admiral Cochrane, Ian Gimble
EDIT: Formatting
Napoleon was kept under strict guard at Longwood House, which was about five miles out of town and all his correspondence and visitors were monitored. There was a lot of rumours about plots to free him, perhaps to go to South America whilst their various independence wars were on-going, but no solid evidence. Bonapartist support was repressed under the restored Bourbon monarchy, so there was a lack of motivation and lack of opportunity- the British would not have let such an important captive go so easily
EDIT: France was also still under military occupation until 1818, and Bonaparte died in 1821 in ill health, so he could never really have retaken France.