Is saying "ah" during supplication between the two prostrations considered an invalidating addition to the prayer, and what is the ruling on stuttering when reciting short surahs?

This addition does not invalidate the prayer because it is legislated. It is better not to try to rectify it, as there are narrations in which the word "guide me" precedes the word "compensate me." As for stuttering, it does not harm the prayer because it is a natural disability that is excused. However, it is disliked for a stutterer to lead the prayer, unless he is leading others who also stutter and he adds an extra letter [due to the stutter].