Is the Imam entitled to ask someone who arrived early and sat in the first row directly behind him to move, claiming that this spot is reserved for the muezzin, and what is the legitimate religious evidence for that?

If you arrive early at a place in the mosque, you have more right to it, and no one has the right to displace you except with your consent, due to the prohibition by the Prophet, peace and blessings be upon him, against making a man rise from his seat and then sitting in it. The mosque is the House of Allah, and people are equal therein. Whoever arrives first at a permissible spot has more right to it.