How are the seventy individuals for whom the martyr intercedes chosen? Does kinship play a role in this, or is the selection made through other means?

The intercession of a martyr for seventy of his relatives is established. "His relatives" generally means his kin, and his wife may be included among them. The entire matter is with Allah; He chooses whomever He wills, and He grants intercession to whomever He wills. That is the bounty of Allah, which He bestows upon whom He wills. There are many intercessors other than the martyr, such as the prophets and the general body of believers, and there is the mercy of the Most Merciful of those who show mercy.