From what I have seen so far here, and from the fact that I've seen you around for years and you are experienced, I do not think the death was your fault. Just like in human babies, sometimes these critters are born with illnesses or defects that are not outwardly apparent.
Given your experience and the fact that the seller was a friend, I do not think the post mortem cross examination you received was necessary. Especially as you kept the seller informed of what was going on and you say you were specifically told not to worry about the setup.
My condolences on the death of your baby snake
