Yes, ANOTHER Stuart TL in parallel with A&D. Though the TL in question is stagnant and co-author is deep into Jacobite research, so I decided to do some PoD hunting on my own.
So, the PoD is 1712, and smallpox epidemics is at large in Europe. TTL it spares Louisa Maria Stuart and Mary Adelaide of Savoy, but kills James III and Mary of Modena.
The consequences?
1. La Consolatrice is now the only descendant of James II living abroad other than Queen Anne, and Anne has no reason to consider Louisa Maria illegitimate. It also gives a potential heiress option other than a Hannoverian to English government.
2. Louis XV will NOT be an orphan TTL, at least the poor boy will have a mother. So he'll be raised differently TTL for a fact.
Its an interesting idea to be sure. As you pointed out Louisa's legitimacy was never in question and she was supposedly looked on with some favor by Queen Anne (well it was claimed posthumously). However, the problems are two-fold. First, Louisa would be in the custody of Louis XIV, and two, she's still a Catholic and therefore excluded from the throne.
Perhaps, if Anne seriously wants her half-sister as heiress, she would demand custody of the Princess as part of the peace deal. Assuming Louis XIV would go for it, that solves the first problem. The second is the real deal-breaker. Do we know much about Louisa's views on religion? I mean I can find a passage from Wikipedia that suggests she didn't care much about religion on some things:
"Louisa felt keenly that Jacobites in exile had made huge sacrifices for her family, and she herself paid for the daughters of many of them to be educated. In this, she made no distinction between Roman Catholics and Protestants, supporting the daughters of both."
But that doesn't mean she'd convert. However, if she does become Anglican, then I think legally she'd be heiress presumptive. The Act of Settlement made the Electress Sophia the heiress to the throne and excluded all Catholics and those married to Catholics. It doesn't say what would happen if a Catholic higher in the de jure line of succession converted to Anglicanism. That grey area means that Anne and her ministers could interpret a conversion to mean Louisa, or anyone else higher than Sophia, to restore their succession rights. Provided they aren't married to a Catholic.
Which brings us to the final problem: Louisa's marriage. Now it seemed to be implied that Anne's idea was to marry Louisa to the Elector Georg Ludwig, AKA George I of Great Britain. This would consolidate both claims, with their children inheriting the throne. Don't know if such a marriage would be realistic however. The other potential candidates I can find are Anne's nephew by marriage, Prince Charles of Denmark (bn. 1680) and Prince-Bishop Ernst August of Osnabrück, the younger brother of Elector Georg (bn. 1672). Other than them the only possibility I can think of would be a domestic match, perhaps to a grandson of Charles II.
