Will the Imam be sinful if he puts forward someone who mispronounces some verses of the prayer other than Al-Fatihah, knowing that there is someone better than him in Quranic recitation and Tajweed, and is this considered an infringement upon the right of the better one?

The Imam should not appoint someone who makes mistakes (Lahn) in recitation, even if it's not in the Fatiha. He is sinful if the mistake alters the meaning. However, if the mistake does not alter the meaning, his action is disliked (Makruh).

The Imam can be advised gently and the mistake of the appointed person can be explained to him. If he does not respond, the matter should be referred to the concerned authority. The questioner is advised not to be eager for the imamate if there is someone else suitable for it.