6 inches off from one corner to another is allot of space and allot of fall and rise.
I am alittle more concerned about the additional weight that you will add with 3/4 inch ply and even that is not going to fix your problem.
You have to consider whether your going to raise areas of the floor or lower high areas. Increasing height is easier than lowering high places, but that means weight and doors and stuff will be off considerably.
Your budget will have to be enormous for this project.
100 years old, she has really set in her ways. You might entertain the idea of adding thicker carpet pads and a thicker pile carpet and let it go as it is.