Nathan;
You are right to show the bottom of the Citation frame and all the rivets. That is what I found necessary to keep the belly pans rigidly attached to the bottom of the frame. I think your approach of welding 16 ga. 4130 sheet steel to the bottom is way less work, and most likely negligible difference in weight.
My hesitation to use welded steel is the difficulty of replacing the pan when it becomes so damaged by banging the ground and sliding over rocks and other things that it has to be replaced. But most of the cars these days have skids of some sort that that is probably not worth considering any longer.
I found that the only way to keep the belly pans attached was with 2" wide, glued and riveted joints. It is a lot of work. But the pan is an integral part of the frame structure and makes a very considerable contribution to the stiffness of the frame.
When I used .060 thick aluminum belly pans, after they had been run for a season, many times the belly pan could not be reinstalled after it had been removed because the holes would no longer line up sufficiently to be reattached with the same sized rivets. With thicker aluminum the belly pans did go back on if they were not otherwise damaged.
Joints are always a design issue. I think the fewer you have the fewer potential problems you might have.
When I was with PacWest I engineered on a Lola T97 that had over 30,000 miles on the tub at the beginning of the season. We banged the walls more than once that year. And we had some very good results as well. That tub had a lot of professional repairs over the years it was run. So, yes, composites will stand up to abuse. But that tub cost more than a FF or FC of that time.
The rules and their interpretations change over time. I have pushed a few rules issues in my time designing cars as have you. I have won a few and lost on a few as well. And the rule book has changes as a result of some of some things I tried. I remember one time when I protested a car for a construction rules violation and he protested me. We were both disqualified and rightly so. The following year the rules did change.
Welcome to SCCA.