So, there are two questions here, related or not related to one another.
First question, say if in 1736, there are a series of tragedies that befall Britain, firstly there are a lot of riots, poor harvests, and a member of the royal family, perhaps the Duke of Cumberland, dies. Over the water, James Francis Edward Stuart, otherwise known as the old pretender also dies. His son, Bonnie Prince Charlie is now the Jacobite claimant, he is recognised as such by France, Spain and Rome. The chaos in Britain, prompts another Jacobite insurrection, which turns out to be successful, Charles is now King, and has converted to Anglicanism. Who does he marry?
Second unrelated question, what would it take for Rinaldo, Duke of Modena to agree to marry his daughter to James Francis Edward Stuart, the Old Pretender?