Is it permissible to lead in prayer a worshiper who joined the Imam only for the Tashahhud (sitting for the final salutation)?

There is no disagreement among scholars regarding the validity of the imamate of one who joins the Imam for less than a rak'ah. The disagreement only concerns one who joins for a rak'ah or more. So, whoever joins for less than a rak'ah, his imamate is valid. He should intend to be an Imam after he has intended to be a follower, because a condition for imamate is that one must not be a follower.