Try a can of Lubro Moly Diesel Purge. Run the engine directly off the can. You'll get a good cleaning and at the same time you can isolate the fuel. By doing that you can see if indeed you got a batch of bad fuel, or gas, mixed in.
Does the car smoke a lot? When I had a bad injector in my 300SD, it shook like it was missing and smoked more than usual at idle. I too cracked the lines one by one and almost didn't notice any difference except in the #5. I pulled that injector and opened it up,the needle was really tight, almost seized. A rebuilt injector cured that.
I'd try the diesel purge first before messing too much with the injectors. Your son could have gotten some really bad fuel at that country gas station. This way you can isolate any bad fuel from the tank and the same time get a really good cleaning of the injectors. Good luck.