No, my side skirts don't provide any support fer the main belly pans at all. The side skirts are basically a gap cover fer the space between the belly pan and the floorboard.
I haven't bottomed out the belly pan so far. With the new version, I won't have to worry about any serious amount of sagging due to the added aluminum angle supports.
I won't be painting the bottom of the belly pan...just the internal side facing the bottom of the floorboard. Besides...if I had to polish the belly pan, I'd just unbolt it from the crossbeams and polish it on the ground versus trying to do that on my back underneath the truck.