Is it permissible to pray behind an Imam who recites "Al-Hamdu Lillah" with a kasra on the hamza of "Al-Hamdu," or with a slight imala (vowel inclination) towards a kasra, for someone who is proficient in reciting Al-Fatiha?

Breaking the hamza of "Al-Hamd" in Al-Fatiha is considered a linguistic error (lahn) that does not alter the meaning, nor does it invalidate the prayer. The prayer of the congregant behind someone who makes such an error is also valid.