Paola Windsor doesn’t have a courtesy title of her own, but she can be called by the feminine form of her husband’s courtesy title. So she’s either “Ms. Paola Windsor” OR “Lady Nicholas Windsor”. If they divorced, then she would become “Paola, Lady Nicholas Windsor”.

Before anyone asks- the reason a Catholic can’t inherit the throne is because the British monarch is also automatically the head of the Church of England (a Protestant Church), and has been since the reign of Henry VIII.