I think the concept that trade has to come through your home capital is a reasonable one. There's nothing to say there's no trade going on over your land borders, just not trade that your government can properly tax - unless it all comes through their capital's warehouses. (I think in this period some of London's warehouses were the largest man-made buildings in the world?)
Iirc (and I probably don't), wasn't the ol'Boston tea party due to the fact that London was slapping huge tarrifs on tea, which came from India via London?
As to whether the trade ports are bugged, that's another matter entirely ;)
PS. I don't think the Irish trade port gets trade income for GB because it's not connected to London via a land route, unlike the Scottish ports. However, I don't know how this applies to say, a pan-European Prussian empire, where nearly all their ports would be connected via land... maybe it has to be in a province directly adjacent to your home province as well?
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