Firstly, Manchester is a very large operation for Etihad, with offices, engineering, crews etc able to handle the arriva, more effectively.
Given the length of flight, the crews needed to rest due to hours, and the contracts set up meant they could slip into a hotel once the medical was sorted. It can not have been an urgent issue as they seemed happy to continue to MAN rather than an urgent divert.
Manchester also has a large hospital on its doorstep, and while we don't know the medical situation, said hospital specialises in Cardio-Thorasic care.
Secondly, the previous passenger didn't die on an Etihad flight, it was an Emirates flight and without knowing the situation we don't know the reason he died before landing.