Leg weakness most commonly arises from nerve compression in the lower back. Hence, pain in the lower back region should not be ignored, especially if it is longstanding. The shooting pain, which travels from the lower back to the knee or the ankle, is because of the irritation of the nerves caused by compression.
Heart disease is a serious medical condition in dogs, cats and humans. Cats do not normally develop arteriosclerosis or coronary artery disease, which are both common diseases in humans. In general terms, heart disease can be divided into two categories, congenital and adult onset forms.
There are many congenital heart diseases/defects, including benign conditions such as atrial septal defects, up to potentially fatal ones such as hypoplastic left heart syndrome. If by heart disease you mean atherosclerotic heart disease which causes heart attacks, then I am not aware of anyone ever being born with it. There are conditions in which one can develop very early coronary artery disease, but that is usually in the 20s, not at birth.