Will the perpetrator of major sins (such as fornication) who affirms the Oneness of God and has not associated partners with Him abide eternally in Hellfire if he dies without repentance, or will he be punished and then be admitted into Paradise?

The perpetrator of a grave sin who persists in it will not abide eternally in Hell, as long as he is a monotheist. However, he is subject to Allah's will; if He wills, He will forgive him, and if He wills, He will punish him for his sins, after which his ultimate destination will be Paradise.