Perspective
OK sticking with large format.
I can see what you are saying about the angle of view.
"When you run a 150mm lens out to 300mm to do a close-up, the angle of view captured on a given format decreases to the same angle as a 300mm lens focused at infinity." But that's irrelevant, as one lens is focussed at 1:1 the other at infinity. However I have realised how you've got muddled.
Forget format lens type etc the perspective is always controlled by the distance of the camera from the object, but the relevant part of the camera is the nodal point of the lens.
So now as you move a 12" lens with your words "let's say 30 degrees corner-to-corner on 8x10. But as you extend it outward to focus closer, the coverage increases and that 30 degrees now covers 11x14, and you're cutting the 8x10 middle out of it." You are also effectively moving the camera nearer to the subject by your actions.
So now two options, the camera stays fixed the lens gets closer as you focus so yes the perspective is changing but actually in the opposite way to your assumption. - ("Perspective becomes like a 17" lens") instead as you say yourself the overall coverage is increasing - a wider perspective because the lens is closer to the subject, the image cropped to the film format.
Second as the camera is focussed if the lens object distance is kept fixed, perspective remains identical.
This is why it's far easier to focus and use a 10x8 or larger format camera with back focussing when shooting close up.
Ian