Maybe I'm misreading something but the way I see walls and floors being installed, ... the only way I can see this happening is if the floor is a separate component from the rest of the house. The 'structure' can be level and plum while the floor is off-level (because they're separate.) On the other hand, iff the walls are supported at the ends by the SAME means as the floor, then EVERYTHING should be out of wack (assuming no foundation problems as you say). Because what I'm thinking is that if the poor workmanship is in the installation of the hardwood floor, then the previous owner must have started with 'thick' flooring at one end and finished with 'thin' flooring at the opposite end that's 1.5" thinner!
What's the floor, foundation, and wall construction in so far as where they all MEET??? What are YOUR thoughts as to what might need to be done to correct the leveling problem w/o having to install carpets? (Aside from what I might think.)