What is the ruling on the imam supplicating after the Friday sermon and the congregants saying "Amen" to his supplication, knowing that the supplication is for seeking forgiveness, healing, mercy, and righteousness for Muslims, and that the Maliki madhhab is followed in this country?

It is from the Sunnah for the Imam's sermon to include a supplication for the Muslims, encompassing the good of this world and the Hereafter, in emulation of the Prophet, peace and blessings be upon him. The Imams of the Muslims after him also used to do this. It is desirable for the congregants to say "Ameen" to his supplication silently.