Is it permissible for groups in tents on the Day of Arafat to have one of their own lead them in prayer, combining and shortening the prayers aloud, similar to what happens in Masjid Namirah, given the difficulty for all pilgrims to reach the Imam?

The fundamental principle is that Muslims should pray behind a single imam, who is the general imam of the Muslims or someone appointed by him to lead the prayer, unless his innovation (bid'ah) takes him out of Islam. If it becomes difficult for all of them to attend the Namera Mosque, they may establish a congregation elsewhere and put forward someone to lead them in prayer and combine the prayers for them. This is supported by the Sunnah (definition), as it is authentically narrated that Ibn Umar prayed at his dwelling in Arafat on the Day of Arafat when the pilgrims delayed the prayer.