Does the prayer of an Imam become invalid if he declines the Imamate due to feeling inadequate, and is he permitted to do so?

The prayer of a less virtuous person in the presence of a more virtuous one is valid if the conditions for leading the prayer are met. This is supported by the Prophet's (peace be upon him) prayer behind some of his companions. There is no harm if the person with a better memory abstains from leading the prayer, as long as someone skilled in recitation is available.