What is Twain's purpose in revealing in this last chapter that Jim has been a free man through almost the entire span of the novel? -chapter the last

Well to begin with, there's that bit of irony in the fact that the entire journey (adventure) really served no purpose because Jim was already free. None the less, if they hadn't run away and taken the trip down the river, Huck would never have had the chance to grow as a person. Nor, would he have the opportunity or the means to move onto the Indian territory as he has chosen to do.