The estate is entailed in such a way that whoever inherits the title also inherits the estate and the man (because as a result of the British adherence to promigeniture it must be a man) who inherits must be the closest living relative by the male line. Consequently Lady Mary or her "son" can't inherit - her fiance, who went down on the Titanic, was also a second cousin and the nearest male relative by the male line to her father so not only would the estate have passed to her cousin but she would also have been his countess. This would have secured the futures of all the females in the family as they would not be "out on their ears" after their father/husband/son had died.
Had the Duke not spurned Mary then she would have married and become a Duchess and again, the future of the remaining family members would have been secured as it would have rested on the presumed kindness of the Duke.
The match with the Duke was definitely the B option - safe but not ideal as Lady Mary's mother's dowry would have remained tied up in the Grantham estate and she would have had to support her mother/sisters/grandmother.