Is it permissible for someone performing a supererogatory prayer to lead someone performing an obligatory prayer, knowing that the incident of Jabir is not considered a case of someone performing a supererogatory prayer leading someone performing an obligatory prayer, because Jabir's intention was the obligatory prayer?

We should point out that it was Mu'adh, not Jabir, who used to lead his companions in prayer after he had prayed behind the Prophet, peace and blessings be upon him. This story was used by great scholars, such as Sheikh Al-Islam, to establish the permissibility of someone performing a fard (obligatory) prayer behind someone performing a nafilah (supererogatory) prayer. The statement that Mu'adh was praying a fard prayer is incorrect, as the Prophet, peace and blessings be upon him, forbade repeating an obligatory prayer twice in one day. He also commanded those who prayed with rulers who delayed the prayer that their prayer should be considered a nafilah, which indicates that Mu'adh's prayer was a nafilah. This issue is a matter of ijtihad (definition) (independent reasoning), and disagreement on it is permissible.