Does one who dies while in a state of major ritual impurity (junub) become a disbeliever (kafir) and exit the fold of Islam, and does everything he touches become impure, and is he prohibited from interacting with people?

The statement that a Muslim becomes an infidel (kafir) if they die in a state of ritual impurity (janabah) is a reprehensible assertion with no basis in Islamic law. A believer never becomes impure, even if they are in a state of janabah. They are like anyone else in terms of social interaction and dealings, and anything they touch does not become impure because they themselves are not impure. If they die before performing the ritual bath (ghusl), they are washed like other Muslims, according to the majority of scholars.