The dragging of the back legs could also indicate a serious spinal or neurological problem. I would agree that an in-depth exam, including blood work, is needed. Is she being treated for the epilepsy? The staring at the floor behavior is consistent with seizures. Of course if her heart is bad, then it wouldn't be supplying the oxygen that she needs and that would cause fatigue and loss of appetite among other things.
It sounds like Abby does have some medical issues going on. I hope that you are able to get some answers for her.
Livin in a Newfie Drool Zone