[...] If Diana was say Countess of Brington.. (is that the name of hte village near to her home?) she would have a settled title in her own right. if she remarried and had more children- they would be Lady Mary X or The Hon Christoper X..I think it woudl have satisfied her to have a title that was hers, and not dependent on being Charles' ex wife or William's mother.
Titles are hereditary for "male heirs of the body, lawfully begotten" unless there is a special remainder in or to the Letters Patent. In normal situations the children of the Countess of Brington with Mr Khan are still simply Mr Khan and Miss Khan. No Honourables and no Lords and no Ladies.
In very exceptional circumstances, to extremely meritorious leaders in war, a remainder has been attached to the first Peer because there was no direct male issue, so that a brother or a cousin, or by lack of these a daughter, would become the second holder of that peerage.
An example: Admiral Horatio Nelson - his peerage passed by a special remainder, which included Lord Nelson's father and sisters and their male issue, to Lord Nelson's brother, William Nelson. Later Lord Nelson's brother was created
Earl Nelson and
Viscount Merton.
An example: Field Marshal Herbert Kitchener - his peerage passed by a special remainder, which included Lord Kitchener' brothers and sisters and their male issue, to Lord Kitchener's elder brother, Henry Kitchener. He became the 2nd
Earl Kitchener and
Viscount Broome.
An example: Field Marshal Edmund Allenby - his peerage passed by a special remainder, which included Lord Allenby's brothers and their male issue, to Lord Allenby's nephew, Dudley Allenby. He became the 2nd
Viscount Allenby.
An example: Air Marshal Charles Portal - the title
Viscount Portal became extinct on Lord Portal's death in 1971 as he left no surviving sons. The other title
Baron Portal of Hungerford, was succeeded according to a special remainder by his daughter Rosemary. However Baroness Portal of Hungerford died without issue, and so also the barony became extinct on her death.
An example: Rear Admiral Louis Mountbatten - the titles Earl Mountbatten and Baron Romsey were -by special reminder- inheritable by the Earl's daughters Lady Patricia and the male heirs of her body or -by lack of heirs- by the younger daughter Lady Pamela. The son of lady Patricia, the current 8th Baron Brabourne, will inherit all titles.
All these men were war heros and all these titles were bestowed on a male. Giving Diana a hereditary title with a special remainder for her Al-Fayed or Khan children, bypasssing her elder sons from her first marriage, seems very unlikely to me. The named gentlemen were most exceptional and of course the Nineties were not the same as the Victorian, Edwardian or Georgian era...