From reading all the posts it looks like you have some pictures. I thought I would add one of the underside of my car. If you notice the water spots on the exhaust. I have already driven mine in the rain. The car is a few weeks old... but so far no rust under it at all. When the exhaust was being installed I took a good look too.
As a ex-mustang owner... that sucks about the rust. I have owned a 92 GT, 93 Cobra and a 94 GT. Non looked like that after years of use. I hope Ford steps up and fixes the problem.
Have you considers a Rust Converter coating. (You apply it and it turns black.) I used it on the floor pans of my 65 Stang. It worked great and the car never rusted there again.
Here is a link to one brand.
http://www.corroseal.com/