What is the ruling on doubting the pronunciation of a word in the Tashahhud or Al-Fatihah, and is it permissible to repeat only that word and what follows it if the doubt occurred after starting the prayer upon the Prophet, or after saying "Ameen," or after beginning the surah that follows Al-Fatihah?

If the questioner is someone who suffers from waswas (obsessive whispering/doubts), then he should ignore the doubts related to his prayer. If he doubts having recited Al-Fatiha or the tashahhud after completing them, his prayer is valid, and he should not pay attention to this doubt, whether the doubt is about a word or a letter, because the default assumption is that the recitation was complete. However, if the doubt occurs during the recitation and before finishing it, he should restart the recitation from the beginning, unless the doubt is recurring daily (i.e., waswas), in which case he must discard it and not pay attention to it.