What is the ruling on repeating the Isti'adhah (seeking refuge with Allah) and Basmalah (saying "Bismillah al-Rahman al-Rahim") before a surah after Al-Fatihah in prayer, and does that necessitate a prostration of forgetfulness (sujud al-sahw) due to obsessive thoughts about intention?

We ask Allah Almighty for a swift recovery for the person afflicted with OCD. The cure for OCD is to ignore it. The intention is a simple matter; merely facing the Qiblah, having the intention for prayer, and then saying the Takbir is sufficient. What crosses your mind about changing an obligatory prayer into a Sunnah prayer is from the effects of obsessive whispers, and it does not invalidate the prayer. Whoever has frequent doubts should not pay attention to them.

As for seeking refuge (Al-Isti'adhah), it is recommended in the first rak'ah according to some scholars, while some recommend it in every rak'ah, and some dislike it in obligatory prayers. Your performing Al-Isti'adhah in an inappropriate place does not invalidate the prayer, nor does it necessitate a prostration of forgetfulness (Sujud Sahw). Your prayer is valid and you do not need to repeat it.