Is it permissible for an Imam to show leniency towards an innovation (such as reciting Surat Al-Fatiha in congregation after the Taslim) at the expense of the Sunnah, especially given that new worshipers have become accustomed to it?

It is not permissible to agree with the people of innovation (Ahl al-Bid'ah) out of courtesy. Allah the Almighty has warned against agreeing with the people of falsehood in their actions, saying: "They wish that you would compromise [with them], so they would compromise [with you]." (Quran 68:9) Al-Tabari narrated from Ibn 'Abbas that its meaning is: "If you would grant them concession, they would grant you concession." The Messenger of Allah, peace and blessings be upon him, said: "Whoever pleases people with the displeasure of Allah, Allah will entrust him to the people. And whoever displeases people with the pleasure of Allah, Allah will suffice him against the burden of the people." Therefore, it is obligatory upon the Imam to follow the path of the People of Sunnah and avoid the path of the People of Innovation. He should not participate in their innovations but rather advise them with that which is better, and hold fast to Allah and be patient. He should also convey to the worshippers the words of the scholars. It is appropriate to offer advice to the Imam and the worshippers with wisdom and good admonition, and to address the matter gently.