Looking good.
Regarding the foot box and rivets, I’d try to just rivet the panels together. If you need to run some into the 2” tube it’s not a problem. I didn’t rivet into the main tubes for the floors but many do from what I have seen.

I’d postpone riveting panels together until you can’t go any further. It’s nice to be able to take it apart if you have to get to something. When the panels are loose you can reach through the frame. Once the rivets are in you are going under and over. That’s when I understood why lifts were so popular.
-Steve