Is there anyone from the Ummah of Muhammad (peace be upon him) who will not remain steadfast during the questioning of the two angels, or is that exclusive to disbelievers?

What Satan whispers and the heart rejects is not considered hypocrisy. The Muslim who has actualized the testimony of Tawhid (Oneness of God) and acted upon its requirements is hoped to remain steadfast when questioned by the two angels, based on the saying of Al-Bukhari: "When a Muslim is questioned in the grave, he testifies that there is no god but Allah and that Muhammad is His Messenger. That is what is meant by His saying: 'Allah keeps firm those who believe, with the firm word.'" Be keen on strengthening your faith and asking Allah for steadfastness, and supplicate: "O Turner of hearts, keep my heart firm on Your religion."