The coroner doggedly sticks to 'medical' issues, probably because without a wreck to examine the chances of things like bird strikes, engine or structural failure etc. cannot be tangibly evaluated.
To add to Kharon's possible causes, one that hasn't been considered is "locked controls". The attitude of the aircraft as described was not a controlled one. The exclamation by the passenger appears to confirm. Not knowing the intercom system fitted, with the PTT actuated from the rear seat, the pilot may have responded on his intercom but his words would not have transmitted to the world. Hence no reaction.
An engine failure would not have locked control input but an abandoned screwdriver or spanner could easily find its way into a compromising position through vibration or manoeuvres. Over 50 years of flying I have experienced 3 such situations and witnessed one fatal accident in a Harvard warbird. In the Harvard case the screwdriver was found lodged in a control bellcrank. The more pilot input the worse it gets. (In some cases opposite input may dislodge)
If CAsA were professional and respected confidential reporting, all too frequent instances of misplaced tools would be reported for all to learn from. However the Australian regulatory attitude, despite the so called "safety" mandate is one of punishment. What's easier than to blame a medical condition without ruling out obvious alternatives.
Hemel with his medical history should nt have been flying. An experienced Victorian pilot who suffered a similar encounter with a hangar door has a lifetime ban from flying, yet he declared all and was cleared by several local and o/s experts in the field, but naturally CAsA AvMed know better. The ex pilot is allowed to fly with a safety pilot however in the event of a seizure would the 5ft 45kg safety pilot be able to remove the patient from the controls. But this is LEGAL.
Not that the loss of even one life is not important ! But when the instances of pilot incapacitation resulting in a fatality are compared to fatalities as a result of a pilot's attitude perhaps this is where attention should be focused. Hempel's was an Attitude problem (no pun intended) S.W.
Empty skies are safe skies!