Many thanks for all of the advice. I went to a local mom&pop body shop and the guy was super-nice. He quoted me $350.00 to weld in new metal under the passenger's seat including the rear seat footwell. He does "rust management" and restoration on a lot of vintage and antique cars so it's not foreign to him. He reminded me that this is a unibody chassis with no real frame to speak of, therefore ANY flex in the metal is going to result in body flex at least to some extent. His personal opinion (which you may or may not agree with) is that fiberglass may seem very rigid after it dries but that it's not as rigid as metal and it shouldn't be used on unibody chassis to substitute for metal.
All in all, I trust him and he's going to seal everything up right afterwards. I'm OK with this solution and this price.
Next is the front passenger footwell. There is rust under there I just don't know how much. I'll need to tear the soundproofing out and take a look and decide again how to deal with it...