I believe you have another door staying shut even though your control valve is working and that door seem to open, the are so many configurations/possibilities, like def/heat, def/vent, just def, just heat, just vent. I'm guessing (kinda) but I bet there is another door keeping the air back, one thing that wasn't mentioned....when you move the selector to change where you want the air to come from can you hear or feel movement in all modes? Hot and cold. I would run it hot try all modes making sure you hear movement at each stage, from the full defrost to full vents to full floor. Another thing I remember we used way back when I was a wrench head (loved it) for a living, I used a coat hangar (make sure it's perfectly straight as to not get caught up on something around a bend) and I made a little ball end on it with black tape and slowly ran it through the outlets to feel movement and maybe to open a door if we are lucky, and if you do happen to open one, keep doing it until you find the valve, linkage or switch that is connected to it. Bingo you've solved the mystery.
Good Luck
Doug