It's the stuff out the front of the carb that makes me think there's too much fuel in there to start with, although the shop I would hope checked all that out. If it doesnlt look like the carb(s) have been off, check your float (compare the bottom one to the other(s)- also check the needle valve. If that isn't closing properly, what happens is the bowl essentially sees fuel pump pressure, and so there's a higher pressure difference across the jet, and so more fuel flows into the engine thru' that carb.
As you say, oily plug would hint fuel is getting in there, but just not burning (or too much fuel to burn). If you have a spark friend, it's a good idea to do a "megger" insulation test both on the leads and across the plugs (connection to threaded bit) A normal multimeter will say that insulation is good but when you chuck a couple of thousand volts at it, you get a whole differnet story. Having said that, swapping them and "chasing" the problem works just as well, but does need some water to play on!