What is the ruling on praying behind an imam who does not articulate the Takbirat al-Ihram properly, and is the prayer of the congregant valid in this situation?

The opening Takbir (Takbirat al-Ihram) is a pillar of prayer, and it involves saying "Allahu Akbar." Mistakes in it fall into two categories: First, a mistake that changes the meaning, such as elongating the hamza of "Allah" to turn it into an interrogative, or elongating the "ba" in "Akbar" to become "Akbar" (plural of drum). This type of mistake invalidates the prayer. Second, a mistake that does not change the meaning, such as pronouncing the word "Allah" with a fatha (Allaha Akbar), or excessively elongating its "alif," or saying "Allahu wa Akbar" by replacing the hamza with a waw. This type does not invalidate the Takbir, and the prayer remains valid. It is obligatory to advise anyone who makes a mistake that invalidates the prayer and to teach them the correct pronunciation.