Great thread! I've often wondered what would have happened in this particular scenario.
In her biography of Marie Antoinette, Antonia Fraser suggested that Maria Carolina was probably her mother's preferred choice for the French marriage. She was only two years older than Louis XVI, and was also a more assertive and intelligent personality than Marie Antoinette. Apparently, Maria Theresa is supposed to have said that of all her daughters, Caroline was the one who was most like her. Caroline would be likely to involve herself in government and ensure that France followed a pro Austrian policy (much as she did in Naples), and she probably wouldn't have found Louis much harder to control than she did Ferdinand of Naples.
If that did happen, then Maria Theresa might have tried to marry Marie Antoinette to one of Louis's younger brothers, thus doubling the chances of having a half Austrian future King of France, as well as giving each sister a support system in the other. In that situation, Maria Amalia would still have been available for the Parma marriage.
If that didn't happen - and France might well have been wary of too much Austrian influence at Versailles, then Antoinette would probably have been married to the Duke of Parma. I'm not sure what would have happened to Amalia. No one seems to have been keen on the Zweibrucken marriage, but if Maria Theresa had daughters in France, Naples and Parma then she might have decided to take a more relaxed view of the situation.
Interesting!