Is it permissible to perform the rituals of 'Umrah on behalf of the Prophet, may Allah bless him and grant him peace, and on behalf of the Companions, may Allah be pleased with them all, and is it a condition to perform a separate 'Umrah for each Companion?

It has not been reported from the Salaf that they dedicated the reward of good deeds to the Prophet, peace and blessings be upon him, or to his companions. The Prophet, peace and blessings be upon him, is self-sufficient, as are his companions, may Allah be pleased with them. What is obligatory is to send prayers and peace upon the Prophet, peace and blessings be upon him, and to invoke Allah's mercy upon his companions.