What is the ruling on insulting the Companions, whether out of ignorance or conviction? And does Mubahalah have a specific formula, or controls and conditions for its use, and does it necessitate punishment for the liar?

Indeed, reviling the Companions is a grave sin, especially for one who knows their virtue. As for mubahalah (mutual imprecation), it is permissible. The Prophet (peace be upon him) invited the Christians of Najran to it. Its form is that the two parties engaged in mubahalah invoke Allah's curse upon the liar among them. Mubahalah should only be resorted to in an important matter related to religion, after establishing proof, clarifying the evidence, and when advice is no longer beneficial. It is a condition that it occurs after establishing proof and striving to remove ambiguity. Some scholars are of the view that it should be with those who fear Allah, and not with people of deviation and hypocrisy.