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...

That is great....

and I do agree with the guy about the Fiberglass... it can be stiffer in and of itself... but that does not count towards the structure of the car in shareing forces..... fiberglass is very strong up to a point.. then it totally lets go... whereas metal stays in there.... and getting the fiberglass to metal joint so it will do the same as putting metal in is impossible in my opinion....

I've had this before, and chances are when you pull away the carpet and soundproofing right up under the dash, put your head in and turn towards the pole star, you'll see it! The battery tray area corrodes, there are some bad traps there; water can run along the cross member and just sit in that area round the bulkhead/firewall.

Here is where a Digital Camera SHINES..... put it up in there instead of you head.... just reading your post made my neck hurt.