Furnace Air Flow

I would start with a borescope inspection of the vents. You can probably "borrow" one from autozone or some other local auto parts store. You will likely have to give them a refundable deposit. Look into the vents to see if you can find a restriction point. Once you find the problem then you will know where to start removing the bolts that hold the coroplast up. They are generally self drilling/tapping and if one breaks off it's no big deal to run a new one in a few inches away from it.
I doubt rigid ducts would collapse. They may come apart. I would guess you have flexible ducts and it may be laying over top of something like a wire or water line. As the flex duct settles it could pinch off the air flow.