Is there a school of thought (madhhab) in Islamic jurisprudence that holds the view that emphasizing a letter in prayer (salat) does not invalidate it, in order to alleviate the whispers of clear grammatical error (lahn jali) which cause difficulty in pronunciation?

Ignore these intrusive thoughts and do not pay attention to them. Maliki jurists do not invalidate the prayer due to a linguistic error ( لحن ) at all if the person making the error is not intentional, whether the error is in Al-Fatihah or other parts of the prayer. According to their madhhab, your prayer is not invalidated. Al-Dusuqi said: "The summary of the issue is that if the person making a linguistic error ( لحن ) is intentional, his prayer and the prayer of those behind him are invalidated by agreement. If he is forgetful, it is valid by agreement. If he is naturally unable to learn, then it is likewise valid because he has a speech impediment ( ألكن ). If he is ignorant but capable of learning, then this is an area of dispute, and the most preferred opinion is the validity of the prayer of those behind him, and even more so his own prayer, due to the agreement of Al-Lakhmi and Ibn Rushd on it." There is no objection for you to act according to this madhhab. All you have to do is recite in a normal, natural way, and then ignore these intrusive thoughts. Know that your prayer, when you turn away from intrusive thoughts, is sound and accepted, God willing. So, do not repeat the prayer no matter how much Satan whispers to you.