What is the ruling on congregational prayer if one of the worshippers in the full first row invalidates his ablution (wudu'), and should someone from the second row step forward to fill the gap, taking into consideration the ruling on a person praying alone behind a row, and what is the optimal course of action in this situation?

If there is a gap in the first row and only two men in the second row, neither of them should move forward to fill the gap and leave his brother alone behind the row. This is because the command to fill the gap conflicts with the prohibition of an individual praying alone behind the row. Prioritizing standing with his companion in the row is closer to correctness and safer for the prayer, as it avoids the disagreement of those who invalidate the prayer of an individual behind the row. Sheikh al-Islam Ibn Taymiyyah preferred remaining in the row despite the gap, because filling the gap is recommended, while forming rows is obligatory.