What is the ruling on one who sometimes pronounces the letter "dal" in Al-Fatiha as a "ta," despite repeating the verses frequently, to the extent that he misses bowing with the Imam? And is this considered a form of waswasah (whispering from Satan/obsessive thoughts)?

If you pronounce the letter "dal" (د) correctly, then it does not matter if the listener hears it as "ta" (ت). However, if you sometimes pronounce it as "ta," try to pronounce it as "dal" in Al-Fatiha during prayer. If you are able to do so, then do it; otherwise, there is no harm. If you can do it slowly, then that becomes obligatory for you in the prayer of an individual (one praying alone). As for the congregant (ma'mum), he follows the Imam and does not delay to recite it, because it is not obligatory for him according to the Hanafis and Malikis, and this is easier for you.

If the matter is a whisper (waswas), then pay no attention to it and consider your pronunciation correct. You can present your recitation to an expert for evaluation. If your pronunciation is correct, then what you feel is a whisper. If there is a defect, try to correct it; otherwise, there is no harm upon you. It is permissible for someone who is like you or less proficient in recitation to pray behind you.