I think the only good fix to this is to replace the tub. I'd call the manufacturer and seek their opinion. If it is determined to be a manufacturing defect, they'll likely replace the tub, but that does not cover any labor or the tile (as far as I know). The rust is the beginning of the end. For max longevity, you can't beat a cast iron tub.
If the floor is not flat, it's possible that applies some torque to the whole tub. You want either have a tub that is stiff enough to support itself (CI), or provide good support underneath it for everything else. Plus, the tub's top surface must be level in both directions to drain well and prevent water from pooling places.