Will a Muslim who dies having committed the major sin of fornication, and without having repented, be saved from the torment of the grave and Hellfire if Allah wills not to punish him, or will he not be saved from any of the stages of punishment?

If God forgives the perpetrator of a major sin, it means that he will not be punished for it, because forgiveness entails the complete removal of punishment. So, whoever's sin is forgiven will not be punished for it. Forgiveness is more complete than expiation (تكفير), and it includes protection and preservation. It encompasses non-punishment in this world, in the Barzakh (interim period after death), and in the Hereafter.