What should a worshiper in the first row do during congregational prayer if his ablution is invalidated, and how should he act to leave the mosque without disturbing the devotion of the other worshipers, especially given their large number?

One must turn away from the whispers of Satan during prayer. Ablution is only invalidated by certainty of the emission of something. If a Muslim is certain that something invalidating ablution has been emitted, his ablution is invalidated and his prayer becomes void, because purification is a condition for the validity of prayer. He must then repeat his ablution and prayer. If he is unable to do so, he must remain without prayer until he can perform ablution and repeat the prayer.