An old topic, but here's my 2 cents nonetheless (and part of my effort to make the board more lively)...
In the Pacific, I don't have such an issue with the convoys being theoretically more than the DEI. The convoy losses there could be viewed as being not just the loss of the resources (oil?) being transported, but also the value of the merchant ships sunk, as well as the resources used in patrolling that area, etc.
With Italy, I think the Med convoy should be a viable target so long as Italy has at least one territory left in NA. Once they are completely out of Africa, I think they have a good argument that the convoy shouldn't be able to be attacked any longer (unless they get back into Africa, which would probably be unlikely). I guess you could do the same for Japan, but as an island nation with few natural resources Japan was far more dependent on the import of food and raw materials than Italy. I really doubt this would become an issue, though, since the Japanese really can't afford not to take the DEI once at war.