Is the prayer considered valid if one is unable to pronounce the letter "rāʾ" correctly in Al-Fatiha and the two Tashahhud recitations, and the worshiper only pronounces the word once, knowing that this difficulty also occurs outside of prayer?

One who suffers from waswasah (obsessive whispering) should not pay attention to his doubts concerning the recitation of Surat Al-Fatihah and the tashahhud; his prayers are valid, and the cure for it is to ignore such doubts. As for one who does not suffer from waswasah, Al-Fatihah is a pillar [of prayer] and must be recited correctly. If one is unable to pronounce a letter in it, his prayer is valid. However, if he does not exert effort despite being able, his prayer is invalid. For the tashahhud, any authentically transmitted formula suffices, and most scholars consider Ibn Mas'ud's tashahhud to be the most comprehensive. If one omits a letter despite being able to pronounce it, his prayer is invalid according to a group of scholars.