The problem that I see if you opt to 'cover over' the existing wall, what will happen behind the 'old wall' when water leaches to it? If the cause of the old wall's movement is due to inadequate drainage, then that won't change if you cover it up. Depending on the amount of hydrostatic pressure, the old wall may or may not move. If you live in a frost zone, the old wall would have to be buried below the frost line. If it's not, it will move under there and affect the new wall.
FWIW, I've been down this road before. I didn't have 3 tiers over 40 feet but I opted to remove the old wall so I could do the job right. I know how you're feeling but I say take out the old wall.