If the father (Prince) does not hold another title, his children will be Prince or Princess. The off spring of that Prince will also be a Prince, but not so with the Princess - especially if she marries.
If Prince William out lives his Grandmother (Queen Elizabeth II) and his father (Prince Charles) and marries his wife would be the Queen of the UK.
Neither, once a prince he can only be king. Actually, a duke is higher in Nobility Rank than a Prince. A prince's title does not change because he marries, however, once he marries, he is usually awarded a dukedom for his parents (King/Queen). As the ruler of this dukedom, he then becomes a duke. An Earl refers to chieftains set to rule a territory in a king's stead.
She could be HRHPrincess William, but she seems to go just by the DUCHESS OF CAMBRIDGE.
HRH Prince William of Wales Duke of Cambridge Earl of Strathearn Baron of Carrickfergus He goes by: William, Duke of Cambridge or Prince William
she marries the prince
It depends. If he is already a prince, then she becomes a princess. Unless the prince also has another title such as Duke or Earl, etc. The wife of a prince that is also a duke would be referred to as a Duchess. The wife of a prince that is also an Earl would be called a Countess.
Harry's wife will become a princess and will take the major title of her husband, depending on what that is at the time For example, when Kate married William, she became a princess, but took on the title of 'Duchess of Cambridge' because the title of 'Duke of Cambridge' out-ranks your bog standard prince title. When Charles and Diana married, Diana took the title of 'Princess of Wales' because Charles was the Prince of Wales, which was a much bigger deal than just your normal 'run of the mill' prince.
This title is assumed by any woman who marries the British monarch's second eldest son, who traditionally always carries the title Duke of York. The monarch's oldest son is always traditionally the Prince of Wales.
Britta marries the prince at the end of the book.
Windsor
Prince Author marries Guinevere.