Does the Imam who forgets while reciting the Quran during prayer sin, given that the congregants insist on him leading the prayer because he is Arab and a memorizer of the Quran, while he does not experience this forgetfulness outside of prayer?

We advise you to review and master what you will recite before the prayer, and to seek Allah's help and supplicate Him to ward off the treachery and whisperings of Satan, who wishes to deprive you of leading people in prayer. We advise you to accustom yourself to khushu' (devotional humility) and to ponder over what you recite in prayer. Among the things that help you avoid error is to recite in prayer that which you have mastered. We advise you to continue leading the prayer and not to yield to Satan.

As for reciting the Quran during ruku' (bowing) and sujud (prostration), or occupying oneself with contemplating what has been memorized during them, it is forbidden, due to the Prophet's (peace and blessings be upon him) saying: "I have been forbidden to recite the Quran while bowing or prostrating. As for bowing, glorify the Lord, the Mighty and Majestic, therein. As for prostration, strive in supplicating."