AFAIK the collapse in supply was historic and a big part of why the NKPA wasn't able to breach the Pusan perimeter. I haven't played this version of the scenario, but it definitely was the case in the older one that shipped with ACOW.
It it wasn't for better/worse than historic performance being able to shift the outcome in phase 3 of the war, I'd almost recommend a scenario that skipped the initial UN route, and the following NKPA route following Inchon; and then begin with the US getting close enough to the Yalu that China was making noises that sounded ominous to everybody but bugout doug.
Did you ever see history portrayed as an old man ... weighing all things in the balance of reason?
Is not [it] an eternal, imploring maiden, full of fire, with a burning heart and flaming soul, humanly warm and humanly beautiful?