If your Dad has 3rd degree heartblock then it would make sense that he is pacing 100% of the time. Complete, or third degree, heart block is a condition whereby no electrical signals from the top part of the heart, the atria, reach the bottom chambers of the heart, the ventricles. Thus, the ventricles must generate their own electrical pacemaker impulse. However, the electrical impulse is much slower than the regular heartbeat. This can lead to a failure of the heart to pump blood to the organs of the body and sudden cardiac death thus the pacemaker is doing it's job.
Chest pain is a common heart attack symptom. Since diabetes can impact the nerves, some people with diabetes have heart attacks without experiencing any pain whatsoever. These so-called silent heart attacks are not usually diagnosed until a routine medical exam.
Anyone who has experienced a silent heart attack is at greater risk for another, more serious heart attack and should be closely monitored by a cardiologist.
You do not mention how bad your Dad's dementia or Diabetes, is also what age is he, and does he live at home ?
The gold standard for coronary artery disease is coronary angiogram, used to identify the exact location of disease, anatomical details and the severity of disease. I would discuss with your Dad's cardiologist if doing further tests will improve his quality of life and if you feel it will after talking with the Dr. then you may have your answer.