Im assuming king William V will make new Letters Patent restricting the use to HRH prince(ss) to children of the main line. He may choose to do so for the descendants of his father Charles III or of his grandmother Elizabeth II. I doubt he'll that the titles from his grandmother's cousins.

He might add that grandchildren of the main line become lord/lady (that would include Peter Philips and Zara Tindal). I don't think it matters much for assisting the crown if you are style HRH princess Zara or Lady Zara as long as people are aware of your line to the sovereign.

That is if William V will feel the need of extra hands. He might still have the Duke and Duchess of Edinburgh and his own children are growing up rapidly. In 10 years from now they will be able to take on some responsibilities if so required. In Spain the bulk of the representation of the monarchy is done by TM the King and Queen and that seems to work. A similar path is visible in Norway, Luxembourg, the Netherlands and when it comes to the younger generation of the Danish Royals.