Is it permissible for the Imam to lead the worshiper in the Fajr prayer again in order to attain the reward of congregational prayer? What is the ruling on performing the Fajr prayer voluntarily during the detested times? And in this situation, should the two rak'ahs before Fajr (Raghibat al-Fajr) be offered before the obligatory prayer or after it? What if there is ample time?

There is no objection to the Imam leading the prayer again for those who missed it, provided that this is not during the prohibited times. This is based on the report that Mu'adh ibn Jabal used to pray with the Prophet, may Allah bless him and grant him peace, and then return to lead his people in prayer. As for the prohibited times (after Fajr until the sun rises, and after 'Asr until the sun sets), it is not permissible for someone who has already performed the obligatory prayer to pray during these times, whether as an Imam or a follower. Achieving a congregation for those who missed the prayer is a charity for which the doer is rewarded.

Regarding the two rak'ahs of Fajr, if the time is so constrained that it does not allow for their performance along with the obligatory prayer, then the obligatory prayer should be prioritized, and the Sunnah prayer should be delayed until after the sun has risen. If there is ample time, then the Sunnah prayer should be prioritized.