You could bring the Far Plane into the equation. It wasn't in the 2E material at all, so the interactions between the Far Plane and the Planescape material is undefined. You could say that the Lady is an Unknowable Lovecraftian Power From The Far Plane With Power Beyone Understanding, and that there are even greater beings where she came from. She lacks tentacles, but she isn't short in power or unknowableness.
Maybe the entire toroidal city of Sigil is a portal to the Far Plane, and the constant tearing down and rebuilding that the dabus do is nothing more than the key needed to open it. When the Streets are Right, Ultimate Horror shall be Released upon the Multiverse. Or something.