Looking through my mom's research notes I find this:
Blood transfusion rejection came about because two witness doctors had a Witness patient who refused blood they believed he needed. In 1945 they wrote to the Society seeking help to persuade their patient that he misunderstood Acts' prohibition on blood. To their surprise the society said he was right, and the Watchtower article that year followed. See July 1, 1945, issue which has, I believe, Franz style.
This came from an interview with one of the doctors involved. Since they have living family, I probably shouldn't include a name. Blood transfusion became a disfellowshipping offence in 1961 through a question from readers in the January 15th Watchtower.
I have no details of the internal discussions. Wish I did.