Was the succession of Abu Bakr, may Allah be pleased with him, based solely on the opinion of Umar ibn al-Khattab, may Allah be pleased with him, and not on consultation (shura) or the consensus of the Muslims? And if that is the case, is it obligatory for Muslims to follow the opinion of a single person, and is the blood of one who withholds allegiance to be shed?

This statement is incorrect. The Companions unanimously agreed on Abu Bakr's precedence in the Caliphate, based on the Prophet's — peace and blessings be upon him — decision to appoint him as imam for the prayer. They accepted for their worldly affairs what the Prophet — peace and blessings be upon him — accepted for their religious affairs, as narrated by Ali. Umar proposed Abu Bakr's leadership, and the Companions agreed with him. There was no one who contended his caliphate. For further information, one can refer to relevant history books and fatwas.