What is the ruling on repeating the recitation of Surat Al-Fatihah and another surah multiple times during prayer, or repeating the entire prayer when one does not grasp its meanings or achieve devotion therein? And does being distracted during prayer reduce its reward by half?

Repeating Al-Fatiha for the sake of attaining khushu' (devotional humility) or re-performing the prayer due to its absence, are matters with rulings detailed in previous fatwas. Interrupting an obligatory prayer after commencing it is not permissible, due to Allah's saying: "And do not invalidate your deeds" (Quran 47:33). If this act becomes frequent for you, it is feared that it might be waswasa (whispers/obsessive thoughts), and you should disregard it. Your understanding that a lack of khushu' is inherent to everyone is incorrect; rather, among people are those who perform their prayer with complete khushu' and it is fully recorded for them. The hadith intended to explain the state of many, not all, worshippers, and it indicates that some among them have the full reward of their prayer recorded if nothing is diminished from its khushu' and its prescribed limits.