So Diana was NOT a Princess officially?
She was Her Royal Highness, Diana, The Princess of Wales, but not Princess Diana.
She held her title as Princess via her husband - she was not a Princess in her own right, as Princess Margaret, Princess Anne, Princess Beatrice, or Princess Eugenie were or are.
After their divorce, she was given special permission to continue using the title ‘Princess of Wales’ out of respect for her position as mother of a future King, but she lost the ‘Royal Highness’.
I have to go or I will be late and that would not do!