Well, the "Merchant's Alliance" (probably) consists of various members of what once was the Great Kingdom; it hasn't made itself visible. The Solonor Compact only sees merchants from its dominions working in the borders of old Medegia. In two years of my campaign, they were never named and Talain sponsored PCs on missions for a hidden master.
You are quite right about the SB - although in my campaign, the SB have not been as visible as they are in the standard timeline (the Wars worked quite differently, though they still occurred). However, greed is a powerful motivator, and the SB have access to spices from the jungles which they're willing to trade _only_ through these ports in Old Medegia.
All of a sudden, Old Medegia has become important again - thanks in large part to the SB.
