msjazz thanks for the post.
This is a difficult situation. Atherosclerosis, the same process that causes the blockages in the arteries of the heart often causes blockages in other major arteries, as the carotid arteries in your mother.
People with significant stenosis of one or both carotid arteries are at higher risk of stroke with open heart surgery. Often an endarterectomy is performed in these situations. More recently percutaneous revascularization of carotid stenosis has been performed.
If your mother truly has both arteries competley occluded, it will make any procedure she would need extremely high risk. Alot of what happens will depend on exactly where the blockages in the carotids and coronnary arteries are and and where the blood flow goes.
A redo bypass operation is difficult enough. You should be seen for this procedure at a large center where they do alot of complex procedures. She may have options to temporize her pain percutaneously, and if not then the complexity of her case can be evaluated by someone who probably has seen or done something similar.
hope this helps.