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?