I've been watching your post hoping that someone will shed some light for you. We just test fitted the rear valance on my '72 Challenger, and I am in the same boat as you. More questions than answers. I'm not even sure what all parts I still need in order to mount the valance and bumper.
I can see that the trunk floor drop downs have three tabs with holes that "somewhat" line up with the holes on the ends of the valance, so I guess that part is pretty self explanatory. However, the mounting hardware kit that I bought only comes with 4 studs and nuts? Seems like there should be more.
I do see two more holes on top of the valance about twelve inches in from either side. I "believe" you are referencing these in your post, as they are drilled into brackets that are attached to the back of the valance. I assume these are mounting holes, but like you, I don't understand what they attach to because they don't reach the cross rail. Is there another bracket that goes between the top edge of the valance and the cross rail? You mention an "inner bumper bracket", but I'm not sure I follow what that would be.
One last curiosity about the hardware kit that I bought is that it also comes with 4 self tapping metal screws. Any ideas what they would be for?