While discussing the possibilities and impossibilities of exchanging Royal offspring and heirs in times of turmoil is amusing, it doesn't bring you much further in search for the truth. What you accept as possible is your point of view and so called proofs usually are pretty flimsy. Looking at the usual suspects, though, might give you some new ideas as to whether it should or could have been done or not.
Louis XVII died in the Temple prison at the age of ten in the hands of his republican prisoners. Louis XVII was exchanged for another boy who died there. Louis XVII was exchanged for the dead body of a boy. What is true and what is not? Marie-Therese was sent to Austria, or was it her half-sister Ernestine? Any of these scenarios could have been contrived at the time. But who would have been interested in such an exchange? And how far would it have been possible to pull off such a stunt?
Getting Louis XVII out of France would have been the prime object of the Bourbon family. Obviously, they failed even if others might seem to have succeeded. Others could be any group of nobles or even republicans disaffected with the bloodbath of the Revolution. Holding on to a possible future King of France would have been the guarantee for high offices in a new regime. Somewhere they bungled that atrociously. Still, the Royal Family never wore mourning for Louis. And neither Marie-Therese nor Ernestine was ever shown the dead body of the boy king.
After the Restoration of King Louis XVIII to the throne of France, he would have been the last to be interested in a reappearance of a King Louis XVII brought up in any which way come to replace him. Monsieur, the brother of the King and later Charles X, would have been of an equal mind as would the Duc d’Angoulême. The Habsburg Emperor was interested in a stable French monarchy; the proprieties of proper succession would have been of no interest to him as long as the country was kept quiet. The English were of a like mind and would have offered Louis XVII exile, but neither standing nor support. The only ones interested in unrest in France were The Netherlands intent on tightening hold on their south-western provinces and possibly a bit more.
Even if Louis XVII had lived, he would have found no supporters in his own French family or from his Austrian relatives. The rejection of all and any claims by claimants of all colors and nationalities was therefore a logical event. If Louis was among them, tough for him, but that was politics. Even his sister, meanwhile married to the Duc d’Angoulême and therefore a possible future Queen of France, would have had no interest in his return to power. Worse, if the Duchesse d’Angoulême was his half-sister rather than his sister, such a refusal would have been a necessity to cover up for herself.
What of Ernestine acting as Marie-Therese, then? Madame Royale would have been an important diplomatic weapon as the only generally acceptable link to the deposed dynasty. If Marie-Therese went mad in prison, she would have been a liability; she would have had to be watched over and kept out of any mischief, and she would not serve in any marriage brokerage; but she could still be a threat to French stability if she fell into the wrong hands. Supplanting her with a girl already brought up as a French Princess would seem like a perfect solution on a no questions to ask basis. If everybody acted as if Ernestine was Marie-Therese, who would dare to doubt her?
As soon as the Austrians had her in their hands, they would be interested in her being the real thing, no matter what. Marrying her into the Habsburg family might one day open the way to the throne of France. The French Royal Family would have had no interest in who was acting the part; the Emperor was their strongest ally in a restoration bid. Her willful marriage to the Duc d’Angoulême would still have been acceptable to Emperor Francis II as it reunited the closest French dynastic lines. That at least was much better than her marrying into another powerful Royal Family.
|Duchess of Angouleme|
If you take together the political interests of the time, an exchange of Royal children becomes a scenario that would have served many interests. Far from being outrageous, such claims might come nearer to reality than historians usually like to admit. Extraordinary times demand extraordinary actions, and the French Revolution definitely was extraordinary.