The one who is most entitled to lead the funeral prayer over the deceased is the Imam of the mosque, whether or not the deceased left instructions that a particular person lead the funeral prayer over him. If the Imam of the mosque gives up that right and allows the one whom the deceased had named to lead the prayer, there is nothing wrong with that

The Sunnah in the funeral prayer, if it is for a man, is for the Imam to stand in line with his head. If he is offering the funeral prayer for a woman, he should stand in line with the middle of her body.