(1) Was the valance painted on the car or separately?
Painted off the car
(2) Did the valance come primed from an external vender or would it have received the same pinkish/redish primer as did the underside of the floor pans?
Primed on delivery, most likely. Primer usually would not match car exactly.
(3) Besides body color overspray, did the underside of valances get any other paint on them? i.e. black-out treatment?
Body color over-spray along edges and maybe a little splattering of sound deadener/sealant applied after assembly
(4) I typically find some undercoating on the bottom side of the valance... how did this happen?
See answer to 3
(5) I have seen some valances with date code stamped into the metal and others without the stamped date, and with a yellow ink-stamped part number instead. Are both of these valid for factory assemblies.
Metal date stamp would be factory correct, the ink-stamped part number was only on service replacements.