Is it Sunnah for the Imam to stand in the middle of the prayer, and what is the evidence for that? And is it permissible to go against someone who starts the row from the right to the middle, so as not to cut off the row?

It is a Sunnah for the Imam to stand in the middle of the row so that the congregants (ma'mumeen) can hear him and follow him from all directions. This is what Muslims have traditionally done. The prayer is valid even if the Imam does not stand in the middle, because standing in the middle is a desirable Sunnah, not a condition for the validity of following the Imam. As for the rows, it is legislated for whoever comes to stand behind the Imam or fill the left side to complete and straighten the row. The closer a person is to the Imam, the better. If the congregants begin the row from the right, it is legislated for whoever comes to draw them towards the middle of the row if possible; otherwise, they should stand with them. The important thing is not to start a new row until the row before it is complete.