I must admit to having believed the "blow on the head" theory up til now, but having just read that thread and that very long post from the neuro nurse, I would say that, in the absence of stronger medical evidence, which I doubt will ever come to light now anyway, I tend to believe that person's theory. Their summary is taken from what Bobber quoted above .....
"... the two causes of death that best fit the scenario are Aneurysm and AVM, both congenital and unrelated to any blow to the head. None of the causes of death from a blow to the head fit the time frames or history we have on Stu. More importantly to this discussion, John was not with Stu in the four months prior to his death. No head trauma causes cerebral bleeding and death that long after injury. Therefore, John could not have been responsible for Stu's death."