Interesting. I read the relevant part of the amicus brief that you linked to. Personally, I don't think this was a breach of legal ethics or outside of the realm of reasonable zealous advocacy. Of course, it would change the equation to include more context to show that the reason, at least in part, that Santinelli opposed transfusion was because of the inhumane methods of collecting blood in those days. But they didn't misrepresent Santinelli's position nor did they omit a part of the actual quote itself so as to change the meaning. Additionally, this quote has nothing to do with their legal argument as it is under the heading that provides a history on the "Christian" reasoning for their belief. Whether there is a legitimate doctrinal justification for refusing blood is not relevant to the court. The WTS also didn't represent a party in this case, so while the same ethical duties apply, an amicus brief is not going to be scrutinized as carefully in most cases.
Edit to add: The brief is also very preachy and spends little time on the legal merits. My guess would be that this was a non-HQ witness attorney who jumped on this and counted about 100 hours of field service time over a few months working on the brief. They probably don't have the resources to put a Bethel lawyer on things where no WTS $$$ is at stake, especially 20 years ago. This of course, was about whether the witness who had a blood transfusion forced upon her would recover money from the hospital for the indignity of saving her life.