What is the ruling if one's ablution is nullified during the final Tashahhud of the Friday prayer and before the Imam's Tasleem? And is it obligatory to complete the prayer with the Imam, or to pray four rak'ahs as Dhuhr? And what should I do if my ablution is nullified during the prayer and I cannot leave to perform ablution again to avoid embarrassment?

If your ablution is nullified during the Friday prayer before the Imam's salutation, your prayer becomes invalid, and you must discontinue it and leave the mosque. You are permitted to feign a nosebleed to conceal your reason for leaving. It is not permissible for you to continue the prayer while in a state of impurity. If you are unable to leave due to crowding, remain seated until you can exit, then perform ablution and catch the Friday prayer at another mosque if possible. Otherwise, pray Dhuhr as four rak'ahs.