Is it permissible to pray in the mosque behind the aforementioned imam, who is characterized by slandering the Companions, altering the facts of the Sirah, defaming people, making mistakes in some historical information, prolonging the sermon and shortening the prayer, and having problems with his articulation points and speed of recitation in prayer?

Prayer in the mosque behind the mentioned Imam is permissible. The Imam's poor conduct or his adherence to a non-disbelieving innovation (bid'ah) does not invalidate prayer behind him. This is because it is part of the creed of Ahl al-Sunnah wal-Jama'ah that prayer is valid behind any Imam, be he pious or wicked. However, it is incumbent upon the Imam to fear Allah Almighty and to be lenient with people, and to busy himself with reforming himself rather than slandering those who have passed. For Allah Almighty said: "That was a nation which has passed on. It will have [the consequence of] what it earned, and you will have what you have earned. And you will not be asked about what they used to do." (Quran 2:134). Moreover, many historical accounts are not authenticated in terms of their chain of narration (sanad).