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Well I ran a similar campain but I didn't use nobles, but most of the PC's were 'merchants' or of some merchant relation.
The setting was a Caravan, called "Bargain Barren" and was traveling from city to city selling its usuall goods. The PC's were a part of the caravan. I had one two that played Merchants, one was a theif, and was working as a 'body' gaurd to one of the Merchant PC's. THe last was a cleric that was traveling with them because he felt a pull toward travel but didn't understand why...he was sorta searching for himself. His monistary paid the Caravan to bring him along till they returned to his home city.
The adventure was dealing with Political Tarrifs and, of course, attempted raids on the caravan. The caravan was quite large and was often under attacked. I made the attacks very symbolic, they all had reasons. There were no just "The goblins are attacking" type things.
One that stands out was a druid had rallied several goblins to attack the caravan. The druid was recently scared by a similar caravan, as they promised to leave the forest in piece, but an accidental fire had wreaked havok on the forest. Wanting to regain balance he felt the need to strike out against the trade road. The PC's job was to find out why the goblins were attacking and figure out how to stop them. The game was highly political and analytical, and had a lot of problem solving. Eventually they figured out the druid was behind the whole thing and confrunted him.
The biggest down side to the campaine (and the reason why we stopped) was that the merchants in the group felt restless during battle. While there was little conflicts taht required weapons to be drawn, the merchant PCs didn't like to sit out. So just make sure that the people you get to play the talky talky funy funy chars are ones that really want to
I tried to revive the campaine by seperating the group into merchants/normal classes. So the merchants got trapped on a pirate ship, and the playing PC's were locked up in the brig. It was the merchant pcs job to rescue the cleric/thief

lol lets just say it didn't work hehe. In the end I think thief had died, the cleric had converted 1/2 the ship to atheists because his complete lack of connection to his God was ridiculous, and both merchants had thrown themselves overboard and tried to make it to a rescue ship

oh dear...it did seem like such a nice adventure on paper...goes to show

a bad DM can ruin even the nicest of adventures LOL
oh well
I hope I helped you a bit hun