This is a fascinating premise, although I would agree that it's important not to end up overplaying the changes that one man could bring - even when that man is the Duke of Reichstadt.
Also, regarding how France under Charles X (if he were still king) might take to the Duke's position in Vienna, I did read somewhere that he rehabilitated Louis Philippe, who was shunned by the rest of the family for his father being an accomplice in the vote to execute Louis XVI, out of a spirit of Christian forgiveness as part of the devout faith he developed later in life (something that very much backfired on his family later, but that need not happen here). So if he reacted that way to the son of Philippe Egalité, that might influence how he'd react to the son of Napoleon Bonaparte.