@oysteilo Hey Oystello, fair question.
First, when we identified the lend lease routes, the issue of whether the SZ was marked a convoy zone was irrelevant. The only relevant question was, was this SZ used as one of the primary lend lease routes. If so, it was included in the objective.
Second, the purpose of the lend lease modification was to boost Russian income, relative to the vanilla game. Adding a convoy lane there would obviously have a reverse result.
Third, with BM, we strove to create a mod that could be played on the original map (including the OOB set) without modification. For this reason (and because we saw no overriding gameplay justification for it), we did not add or remove convoy zones.
Hope that helps!