Several things rule out a subdural bleed as the cause of Stu's death. A subdural is easily identifiable on autopsy and the blood clot is found on the surface of the brain, not in the ventricle. There is no evidence that Stu experienced progressive deterioration. Death from a subdural would have resulted in Stu being confused and unable to walk several days before his death, then difficult to wake up, and finally comatose for several hours to a day or more before his death. It is inconcievable that Astrid watched him slowly deteriorate to a coma before deciding to take him to the hospital which is what she would have had to do in order for him to die enroute.
All reports are of episodes of headache.
Any headache caused by a subdural would be nearly continuous and they are not generally reported as severe. This is again outside of the necessary time frame for John to be implicated in Stu's death if it were due to a subdural bleed. A severe blow to the head can cause bleeding deep into the brain, fequently with rupture into the ventricles. Death can be rapid, but the patient may last up to about 3 days and during that three days the patient is near comatose or comatose. This fits the situation only if the fall down the stairs occurred just before he died.