Is it considered a sin to act upon the opinion that the takbeerat al-intiqal (transition takbeers), tasmee' (saying "Sami'a Allahu liman hamidah"), and tahmeed (saying "Rabbana wa lakal-hamd") are sunnah, especially in cases of waswasah (obsessive whispering)?

There is no sin in acting upon the recommendation of the takbirat al-intiqal (transition takbirs) and tasmee' (saying "Sami'a Allahu liman hamidah"). This is the view of the majority of jurists from the Shafi'i, Maliki, and Hanafi schools. The opinion of most jurists is that takbir in prayer (other than takbirat al-ihram) is a Sunnah, and prayer is not invalidated by intentionally or unintentionally omitting it. This is emphasized for the person afflicted by obsessive doubts (waswas), as it is legislated for them to adopt the easiest opinions.