What is the ruling on the prayer of one who doubts the number of rak'ahs, and is he required to perform the prostration of forgetfulness (sujood al-sahw) if he doubts whether he performed the first prostration of the prostration of forgetfulness, or whether he observed the tranquility in it, or whether he uttered "Subhana Rabbi al-A'la" therein? And is it obligatory to repeat the tashahhud if he performs a forgotten rak'ah after the final tashahhud? And what should one do if he doubts the takbeer of transition after the prostration of forgetfulness?

A person afflicted with waswas (obsessive whispering) should not pay attention to it. Performing one rak'ah and then prostrating for forgetfulness (sujud al-sahw) before the taslim is valid, but a person afflicted with waswas should not give in to their waswas. Doubt about sujud al-sahw after the prayer has no effect, because doubt after completing an act of worship has no effect, and this is to ward off waswas. If you forgot the second rak'ah and performed another rak'ah in place of the forgotten one, then there is nothing upon you except sujud al-sahw, which compensates for the first tashahhud that was inadvertently omitted. Doubt about omitting the takbirat al-intiqal (transition takbirs) after completing the raising (from ruku') does not necessitate returning to it. The takbirat al-intiqal are Sunnah according to the majority of scholars and obligatory according to the Hanbalis, and your prayer is valid.