#1. I don't recall any staples around the perimeter of mine, but I've heard some talk of some material like wind lace between the trim and the body.
#2. Yes, the slots along the bottom edge is for defrost vents. There are slots in the dash panel at the bottom edge of the windshield too. And if you have a good heater there may be duct boots screwed to the bottom of the dash under those slots that would connect to the heater with a hose.
#3. I don't have an exact size for the screws, but they are like a Trim screw with a small bugle head. A rough guess would say they're about a #8 X 1-1/2". Maybe someone has one that they can measure, or share a picture? I'd have to try to remember to pull one out tonight to contribute that info.
#4. I would... As I said, mine didn't have any staples and I haven't put anything back there, but yours must have had something attached to it as a filler.