Every, and I mean, EVERY bit of that floor pan is "structural". Unibody, remember?

POR15 and fiberglass is fine to patch holes but what you are working with needs to have metal welded/epoxied/riveted (I don't feel like going into that argument again) into place.

Think of your chassis as a cardboard match box (and in fact if you own a Yugo it's an even more apt simile). Pretty stable right? Now, slide the match tray out of it's cardboard shell. Squeeze the shell and what happens? When you remove the floorboard from the unibody equation you are doing the exact same thing. You can roll the empty shell down a flight of stairs (just like driving down a bumpy road) and it will flex and keep it's basic shape. Now, drop a book on the shell which is the equivalent of having a car crash into the side of you.