Before you do anything else, I would give it a hard prune and most of all a good watering. Although they are tough as boots the roots are competing with the footings of your wall for moisture. Having said, they are brutes and tend to take over so it might be better to replace it with something else, that is if you can get the stump out.

It could be a fungus called Verticillium. It is in the soil. I don't think there's anything you can do to save it. You'll have to dig it out and replace the soil if you want another one in that position.

Thank you