Trade is still a feature that causes me to wonder...

I was playing Britain on M/M and by 1746 the situation was as this:
This time I followed the advice of several fellow posters on here and concentrated on the North American continet right from the start. This meant, after destroying the Carribean pirates, I took on the Cherokee nation (destroyed) and the just emerging US (detroyed). 13 colonies were still my protectorate (13 colonies at that point consisting of New England, New York, Virginia, Pennsylvania, Maryland + Kaintuk Territory and Cherokee Territory (which I gave to them hoping that this would boost their income). The 13Cs had one trading port in New York.
I was owning Lower+ Upper Lousiana, Michigan Territory, Algonquian Territory, Carolinas, Virgina and Florida.
I had Commercial ports in Lower Lousinana and Georgia
In addition I had Bahamas, Jamaica, Windward+Leeward Islands + Trinidad&Tobago withwith commercial ports in the first 4.
In Europe I had England (with commercial ports in Newcastle, Bristol, Liverpool and Greenwich), Scotland (cp in Glasgow) and Ireland also with one commercial port.
I had farms, plantations and trade fleets aplenty + some long time trading partners.

Now the following happened - France declared war on the 13Cs - to "protect my protectorate" I joined in the war. A French fleet was blockading New York port, which resulted in ZERO trade between me (Britain) and the 13Cs - strange thing, since we were sharing at least 8 land borders. So, I came to the conclusion that the game recognises only sea trade as trade if the 13Cs are your trading partner, meaning no matter how many land borders you share in North America, trade between Britain and the 13Cs is only taking place between the 13Cs ports and Britains ports. Well, I thought this a strange thing, since the 13Cs could transfer their goods via land to my ports in North America (which were not blockaded by the French) and from there to Britain. Maybe a bit too complicated I thought.
So as not being in danger of bancruptcy, I requested peace with the French (successfully) - which of course did not stop my protectorate and the French from continuing their war. (sighs at the fact that you cannot have any influence on the policies of your protactorate).
Since I didn´t want to lose more money, I cancelled my trade agreement with the 13Cs (New York was still blockaded by the French and I wasn´t making any money from trading anyways).
Next turn I still needed money, but could not finde a trade partner to replace the 13Cs. So I declared war on the French, trying to get rid of their fleet that was blockading NewYork.
No something really ridiculous happened: I French fleet was blockading Bristol - and BINGO I was bancrupt - as having Bristol blockaded ALL my trade was cut of - meaning ZERO income from trade.
Meaning all my other commercial ports had zero value, all my trade cut of just because one port being blockaded????????????????????????????
I checked, double checked and triple checked - NO other sea trade routes were raided, none of my trading partners ports were raided or blocked.
I think it is a bit strong if ONE of your ports being blocked results in bancruptcy.
Whats the use in having more then one port then if - following the games (il)logic - all your trade is going via one port anyways.