HCM can be either genetic or sporatic. My daughter had a severe concentric HCM and no one in the family (either side) has the disease. The fact that your mother has family members who all died around the age of 50 from heart attacks tends to make me really suspicious of HCM. All people belive people die suddenly from heart attacks, actually people die suddenly from cardiac arrhythmias. Chances are the family members died as a result of arrhythmias; that is the cause of death in HCM patients if they die suddenly. At that rate, I would CERTAINLY seek another doctor, try a University Hospital, and tell them about the number of early age deaths in the one side of your family.
Isn't hypertrophic cardiomyopathy a familial disorder? No one else in my immediate family has this problem. My mother's family has one branch with several members who died around age 50 with sudden heart attack, but they all also smoked. Only 1 cousin has had a major heart issue, and he has 4 blocked arteries. My father has atrial fib, so his atrium is enlarged, but he is 80. I'm 58.
There is a possibility that you may have Hypertrophic Cardiomyopathy which would explain the relaxation problem along with the enlarged atrium. It would also explain the ischemia patterns. I wouldn't let the doctor dismiss this, he he continues to do that, I would certainly look for another doctor!