What is the ruling on praying behind an Imam who pronounces "alladheena" (الذين) by changing the "dhal" (ذ) to a "dal" (د)? And what should the worshiper do if it is not permissible or if he doubts the Imam's recitation?

If you have doubts about the Imam's linguistic error (laḥn), do not pay attention to the doubt, because the default is the correctness of the Imam's prayer and recitation, unless you are certain that he is making a clear linguistic error (laḥn jalī) that invalidates the prayer.