Is the imam's changing of the vocal layers of his voice during the prolonged vowels (mudud)—especially in the word "الضالين" (the misguided)—by expelling air or as if the letter 'H' (hā’) intersperses the prolongation, considered a clear mistake (laḥn jalī) that invalidates the prayer?

The change in vocal registers during the madd (lengthening) that you mentioned does not necessitate adding a letter to the word, nor is it conceivable to produce the letter hāʾ during the madd. It appears that this is merely a waswasa (whispering from Satan), so we advise you to disregard it. The prayer is not invalidated unless it is confirmed that a letter has been added to Al-Fatihah. However, in cases of doubt, the prayer is presumed to be valid.