I think the best way to deal with convoys is to have a zone like in AAP and AAE. It is a convoy route for a selected country. If it becomes occupied by an enemy then that country loses income for every turn thereafter until that evemy ship leaves the zone or it is destroyed. If the enemy ship leaves then the route opens again, it should not stay closed if the enemy then leaves! You should not have to have a friendly warship there to keep it open, only to defend it. To sum up what I said: if the zone has nothing there then the convoy is open and adding to the income of the country, if it is closed by and enemy ship then it won’t add to the income of the country any more.
So from my understanding if a convoy route is captured why are we losing money already collected? How does that make sense, the ships delivering IPCs already got to us, thus the collect income phase at the end of the turn. For every turn after then the income should be docked, but why immediately?