Is it permissible to pray behind an imam who does not recite the opening supplication (Du'a al-Istiftah) or the Basmala, repeats the letter "ra" in the word "Ar-Rahman," and does not complete the Taslim by saying "wa Rahmatullah"?

Prayer is valid behind an Imam who omits the Sunnah of the opening supplication (Du'a al-Istiftah), or an obligation like the Basmala according to those who do not consider it obligatory, or who restricts the salutation to "Assalamu Alaykum" without adding "wa Rahmatullah." This is because differences in madhhabs (schools of thought) do not prevent the validity of following an Imam. The opening supplication is a Sunnah, the Basmala's obligation is disputed, and adding "wa Rahmatullah" is recommended according to the Shafi'is but not legislated (masnun) according to the Malikis. However, it is not permissible to pray behind him if you are certain that he adds a letter to Al-Fatiha. As for doubt about this addition, it should be disregarded. He should be advised by clarifying the error so that he may accept the advice.