A few clarifications. Does being out of supply have two negative effects: direct penalties to org, movement, and attack/defense ratings and also increased attrition, which in turn impacts fighting strength? Does attrition only destroy equipment, or does it lower org and/or manpower?
Is the supply bonus from the capital, or other major city, just a constant bonus? The supplies themselves don't have to move, do they? So if I've got a nice, high infrastructure route to my capital, I will constantly get the same supply bonus, even if I also have another army fighting on another front?
I assume neighboring supply zones only offer their supply if it is not being used by units locally. So if I fill each zone along the front to its supply capacity, there won't be any spare capacity to supply extra units in one zone. Do zones only share with their direct neighbors, or can a zone pull from one neighbor and then offer those supplies to another, on the other side?
Finally, can convoys increase the supply available to zones already connected by land? For example, if Turkey were in the Axis, could Germany send supplies across the Black Sea to support the invasion of the USSR? Would it increase the supply of that zone only, or will it also increase the supply of neighboring zones? Will controlling multiple ports increase the supply available to an overseas invasion force?