What is the ruling on the prayer of someone who recites the phrase "Maliki Yawm Ad-Din" with a kasrah and tanween, and is it permissible for congregants to follow an Imam who commits a grave error in his recitation, such as substituting some letters?

Reciting "Maliki" with a tanwin (māliki instead of māliki) is a subtle error (laḥn khafī) that does not alter the meaning or invalidate the prayer. However, substituting one letter for another in Al-Fatihah is a grave error (laḥn jalī), and it is not permissible to pray behind someone who does so, although some scholars have been lenient regarding letters with close points of articulation that are difficult for the common person to avoid.