What is the ruling on prayers performed behind an Imam who was heard sometimes reading "ihdina" (اهدنا) with a kasra on the hamza, and "al-maghdubi" (المغضوب) with a very faint ghayn that is barely audible, and another who pronounces the letter dal in "al-din" (الدين) between the makhraj of the dal and the taa? And should the Imam be asked about his manner of pronunciation?

It is apparent that the questioner is afflicted with some waswasah (whisperings/obsessive thoughts). He must therefore disregard them, avoid them, and not yield to them. The default is that the Imam's prayer is valid, and one should not pay attention to doubts and delusions unless one is certain that the Imam mispronounced a letter from its proper articulation point and substituted it with another.