Thanks Alice. So moving the door to almost closed greatly reduces the opening, which increases the air velocity, which increases the pressure acting on the door. Also moving the door to almost closed increases the effective area of the door on which that increased pressure acts. So the two effects multiply each other proving that a door in a draught that doesn't move when it's wide open, will probably be slammed shut if you almost close it, which means that the witness is telling the truth.
What I am actually getting around to saying is unless you can replicate precisely
all the conditions obtaining at the time and have all the dimensions of the appropriate features then mostly it is going to be guess work.
Someone with a knowledge of meteorology, fluid mechanics and mechanics will make a more informed guess that someone who hasn't.
In so far the door will tend to shut faster the larger the area of it that is exposed to the force the witness is correct. Hence my comment on cos theta.
For the door to "blow" closed there must be an area of high pressure one side of the door and low pressure on the other side QED. For a fixed flow at a fixed upstream pressure the velocity through a small aperture will be greater than the velocity through a large aperture however a smaller aperture will cause more resistance therefore restricting the flow if the upstream pressure remains constant; but just how much air do you calculate is being moved to balance the pressure?
Better stop here it will not tell us much.
Just stick to the high level stuff it is more informative.