Is Mubahala exclusive to the Messenger (peace be upon him), or is it for Muslims in general? Is it permissible for Ahl al-Sunnah wal Jama'ah to offer it to deviant sects? And is it confirmed that a sign will appear to manifest the truth when it is performed?

Mubahalah is not exclusive to the Prophet, peace and blessings be upon him, as some Companions like Ibn Mas'ud and Ibn 'Abbas called for it in matters of jurisprudence. It is permissible for the People of the Sunnah to perform Mubahalah against the People of Shirk and innovation after establishing the proof, offering advice, and when that advice proves ineffective. It is not necessary for a sign to appear indicating the falsity of the one in error, for Allah grants respite and delays.