Because of Melville's experience aboard sailing ships, it is likely when "con" came to mind its meaning related to skillful piloting and not a scam. A sailor of that era would have been familiar with the term Conn, Con, or Cun as defined for example in Admiral Smith's <The Sailor's Word Book>. "This word is derived from the Anglo-Saxon conne, connan, to know, or to be skilful (sic) The pilot of old was skilful, and later the master was selected to conn the ship in action, that is to direct the helmsman.. The quarter-master during ordinary watches conns the ship, and stands beside the wheel at the conn, unless close hauled, when his station is at the weather side, where he can see the weather-leeches of the sails." London Blackie and Son 1867
As a contemporary traditional sailor, the same term applies to my experience and to my first take on the meaning of con..
The OED lists "Con, Conn V2 from 1626 "to direct the steering of a ship" and "The action or post of conning a ship, steerage."
Webster's New International Dictionary 1934. :"To conduct or superintend the steering of a vessel".
The people being conned in the CM are aboard " the favorite steamer Fidèle," translated from the French would be "faithful." Into a cast of 19th century Americans the reader is introduced to a stranger and also to a warning from authorities of "a mysterious impostor . . .an original genius from the East." The key point being there was a lack of specifics as to what is his genius originality but a careful physical description of his person was posted by the vessel's authority. The first of many "strangers" offers Biblical references to "charity" based on St Paul's Charty as the greater than faith and hope. So it appears to me the confrontations between the characters is an examination of how we steer our relationships. Some are guided by faith, some by hope, some by charity for those who put their confidence in their own original genius.. There are also those who do not have confidence in their fellow man and it is they who are conned by their own impostor who doubts and fears based on beliefs that come from outside authorities.
Melville offers many examples of how we conn our relationships. Every reader will identify with those characters who mirror the source of ones confidence. A skillful con can lead to the freedom of enlightenment or to agony of having been conned by an impostor. The fact that every confidence man knows is "the impostor who cons us is our own inner one."