Should one not pray behind imams who are not proficient in the rules of Tajweed and mix up the letters الضاد (Dhad) and الظاء (Zha), or should one pray alone, or should one seek an imam who is proficient in Tajweed, even if it involves some difficulty?

Prayer is not invalidated by a defect in tajwid unless it is a clear mistake (laḥn jalī) that alters the meaning in Al-Fatiha. Many scholars have eased the pronunciation of the letter Dad (ض) as Zha (ظ) due to the closeness of their articulation points, and some have extended this ease to any two letters whose articulation points are close. It is preferable to pray behind an imam who can distinguish well between the two letters. However, if such an imam is not available, there is no harm in following one who cannot distinguish them.