Does unintentional, prolonged pronunciation of the letter "R" during prayer—such as in the Takbir or the word "al-Sirat" in Al-Fatihah—invalidate the prayer if it results from difficulty in articulating the letter "R"?

If one's pronunciation is sound, but they substitute one letter for another or drop the shaddah (doubling) of a letter, their prayer is valid if they are unable to learn correctly. Otherwise, their prayer is invalid. However, if one cannot pronounce a letter due to a congenital defect, there is no blame upon them. Doubling a letter that is not meant to be doubled, without a reason, is a misuse but does not invalidate the prayer, unlike the opposite which does invalidate it. This applies to the takbirat al-ihram (definition) (opening takbir) and Al-Fatiha. If one lightens a doubled letter in Al-Fatiha or substitutes one letter for another, their recitation of that word becomes invalid. A linguistic error (lahn) that alters the meaning in prayers other than Al-Fatiha does not prevent the prayer's validity or the validity of following an imam, unless one intentionally commits it, in which case their prayer becomes invalid. One must be cautious of obsessive doubt (waswasah).