I don't want this to become a medical forum but
Medicals don't prove a pilot is fit. They merely seek to identify pathology or possible risk and by grounding or putting limitations on those involved seek to ensure those pilots flying legally are safer than if there were no medicals
Like other 'facts' on this thread I take the suggestion that his illness was wrong with a pinch of salt. We can't prove someone didn't have a TIA. It may simply be that nothing was found and after the two year grounding he got his medical back. This happens in most cases but he may well have had an event where the blood supply to part of the brain failed
As for the doctor getting in the aircraft, well we medics are as mad as anyone else. We believe we can walk on water and talk to God (whereas surgeons believe they are God). Most doctors would have jumped at the chance of saving a life and being a hero, either ignoring the pilot's medical or believing any problem wouldn't happen on the that flight. The doctor involved may have done a full risk assessment and examined said pilot before boarding, or may not have