To what extent is the hadith true which mentions that the Romans will slaughter the Arabs in the land of the West, and if it is fabricated, is there an authentic hadith that replaces it?

There is no Hadith (definition) indicating that Muslims will never conquer the Romans. Rather, what is established in the authentic Ahadith is that Muslims will conquer the Romans, inflicting a great slaughter upon them in a major battle, and will conquer Constantinople. Furthermore, Muslims are most deserving of the Prophets and Messengers because they have believed in all the Books and Messengers, and have been able to defeat the forces of polytheism and misguidance, which confirms the elevated status of Islam and its adherents until the Day of Judgment.