I think you're right about sticking with a single world market - there's no point for excessive complexity. But, on the note of embargoes, I do have a question:
I'm playing the United States, and the Confederates secede. I declare war. Will I be able to choke them by imposing a naval blockade, or will they be able to access the world market freely even when I've got them totally surrounded on land and sea?
I mean, I understand what you're saying about embargoes not happening too much in the 19th century - cutting off trade with one country just resulted in the same goods coming through via another nation. But cutting off a particular country from the world market as a whole - that was a pretty crucial factor in the American Civil War, and the Germans attempted something similar in WWI as well. So, is there any hope on this count, or will wars have no impact at all on trade?
Well, one could model the overseas trade in the same way Imperialsm has done it (for those of you who don't know what that is: It's a simpler, more streamlined version of vicky, sort of, set in roughly the same time period).
Short primer on Imperialsm's economy.
1) Only the (location of) the capital matters.
2) any resource linked over land to the capital cannot be interfered with
3) for all other trade, each nation builds it's merchant fleet.
4) the merchant fleet is deployed automatically to transport all of your purchases to your capital.
5) the merchant fleet contains of individual ships, which can be targeted by a fleet blockading a port. Not targeted in the sense that you can pick and choose, but each merchant vessel running the gauntlet has a chance of being sunk by a blockading fleet. The chance being determined by the quality, range and firepower of the warships, and the speed of the merchant vessels.
The huge difference with vicky, of course, is that Imperialism was turn based. Each turn represents 3 months of passed time. So, the game didn't have to track merchant vessels. Each turn they sailed, did their business, and either returned or were sunk.
The common factor is that Imperialism, too, modeled one huge world "market". Each nation trades with any other. It's not localised. Not geographically anyway.
What Frog City (the creators) had added, though, was a diplomatic rating between nations. And with that, you could control (albeit roughly) with who you usually traded. Each nation sold to the one it most liked. And then worked down the relations list until everything was sold, or noone was left who would buy. By granting trade subsidies, and through those, by the total trade volume, you could make others like you more, and thus trade more with you than with your competitors.