Is the reward of the first row written for one who came to it, then remembered he was not in a state of ritual purity (wudu'), so he broke his prayer and performed wudu', then prayed in the last row?

"Interrupting the prayer and leaving the row due to the loss of a condition for the prayer's validity is the correct action. It is hoped that whoever arrived early to the first row and then an unforeseen impediment arose, beyond their control, will still receive the reward of the first row. This is based on the Prophet's (peace be upon him) saying: 'If a servant falls ill or travels, what he used to do when he was a resident and healthy will be recorded for him.' This applies to someone who was performing an act of obedience and was then prevented from it, provided their intention was to continue it. This is supported by the hadith: 'Whoever performs ablution perfectly, then goes and finds that people have already prayed, Allah will grant him a reward similar to that of those who prayed and attended it, without diminishing anything from their reward.'"