What is the ruling on the imamate of an imam who substitutes some letters for others, while there is a congregant who is better than him in recitation and memorization of the Quran, but who has not disclosed his proficiency and memorization?

The correct view is that the imamate of a person with a lisp (althagh) is valid. Preference should be given to the best reciter (aqra') then the most knowledgeable (a'lam) in the rulings of prayer. The term "aqra'" refers to the one who recites best, not the one who has memorized the most. It is not permissible for someone who recites well to conceal themselves and put forward someone who does not recite well. If a mosque has a regular imam, then his imamate is given precedence.