What is the ruling on praying behind an Imam who pronounces "Alhamdulillah" as "Alhamdu-r-llah" (with an elongated "dal" and a shaddah on the "ra"), and is it permissible to pray behind him when no other Imam is available?

The aforementioned utterance is an error and a grave linguistic mistake that alters the meaning, and it is unlikely that an imam would recite it this way. The majority of scholars hold that reciting Al-Fatihah is a pillar of prayer, and whoever commits a grave linguistic error in it, their prayer is invalid and it is not permissible to pray behind them, because it is not permissible to pray behind someone whose own prayer is invalid. Therefore, it is not permissible to pray behind someone who recites Al-Fatihah with this mistake. One must be diligent in praying behind an imam who recites Al-Fatihah correctly, while being cautious of falling into whispers (waswasah).