Firstly, are you based in the uk or usa....your term of autopsy indicates USA....in which case i do not know what the anser to your question will be...
If in the uk....and your dad had no hospital treatment or had seen a doctor for some time before his death, then I am sure that a post mortem would had been carried out...
On the subject of the life cover, if the heart attack was unrelated to diabetes, then the policy should pay out, unless specifically excluded on the policy document.
If you are in the uk, then you should complain and appeal to the life company in question. If you get no joy with them then you have the right to complain to the Financial Ombudsman Service.
http://www.financial-ombudsman.org.uk/consumer /complaints.htm
I fear that without a Post Mortem report proving that the cause fo death was unrelated to Diabetes then you may be fighting a losing battle. But for the sake of your dad, you should explore all avenues.
Good luck