With regard to the muezzin standing behind the imam, there is no specific Sunnah that is proven from the Prophet (blessings and peace of Allaah be upon him). The only thing that is narrated concerning that is that it is mustahabb for those who are most wise and dignified to be closest to the imam. The Prophet (blessings and peace of Allaah be upon him) said: “Let those who are most wise and dignified be closest to me”. Narrated by Muslim (432).
But there is nothing wrong with leaving a space behind the imam for the muezzin to stand when he has finished reciting the iqaamah or call immediately preceding the prayer, so that he can find a suitable place in which to pray without having to cut through the rows. And there is nothing wrong with this place being slightly to one side of the imam, to the right or left.
And Allaah knows best.