Is the Imam sinful for putting forward someone to lead the prayer who mispronounces verses other than the Fatiha, when there is someone present who has a better memory and recites with better Tajweed? And is there, in that, an infringement upon the latter's right? And what should he do?

The regular Imam of the mosque has the most right to lead the prayer. If he delegates someone else due to an excuse, then his delegate has more right to lead the prayer than those whom the Imam has not delegated. He should choose the best person. The prayer is valid if the delegate makes a mistake in recitation (lahn) in something other than Al-Fatihah. It is advised to counsel the Imam and discuss the matter with him to avoid disagreement.