i also read, though now i stupidly did not bookmark the link, that his name was marion monroe martin, that he left home to join the war(?), & took the name marcellus monroe to avoid his parents finding him & bringing him home. he was known as major monroe for a time. later he went back to his real name.
this does make sense, though i have no proof yet.
my problem right now is that he is listed (on his son's death certificate) as james m. monroe. where the james comes from is a mystery.