It must be very distressing for Ian's family to authorise a third post mortem, and it is puzzling that the second one, by a competent pathologist, was not definitive.
We should be interested in Ian's state after being hit and pushed over. His head his the pavement with force, though not enough to knock him out. After though, he seemed "dazed". Part of this may be due to alcohol, but a bleed in the brain could do this too. Abdominal bleeding is usually extremely painful, but I have not seen any eyewitnesses describing a reaction to pain in the abdomen or back.
We have to find out whether there was any bleeding into the brain, whether visible, or microscopic.