Bingo! Assuming the adoption was in order, Soetoro remained the legal father of obama even after the divorce. He is still obama’s legal father. Obama may very well have gone to the courts to correct his birth certificate but it amount to a name change only.
There is another possibility with regard to Jr using the name Obama after returning to Hawaii and that is if his father undertook some sort of court action while he was there in 1971-72. If such an action did take place it would mean both Jr’s original and his second birth certificates would be sealed.