When you changed the mix screw, did you check the spring ,washer and O-ring that's placed on it ,some times the O-ring can be damaged by age.  If you slow the idle down so the bike is barely running be for adjusting the mix screw, you should notice the change in rpm.
     the only other thing I know of that could cause you to run rich and foul the plug, besides the float level,  is if the brass washer is not in place between the needle jet and the main jet. This washer holds the needle jet (cylinder) up fully in the carb body , if it's missing the jet will fall away from the body and allow extra unmetered fuel to flow around it .
       You should start keeping tract of your mileage, If your mileage is around 50 that's normal for the jets your now using, if it's low 40's,  your plug is still dirty,your getting extra fuel from somewhere and it has to be the needle jet /jet needle as the pilot jet only controls fuel at idle , and the main jet doesn't come into play until you reach 3/4 to W.O.T.

You have to slow the idle speed down (close the throttle by using the knurled screw on the left side of the carb ) to allow you to hear the change in rpm when adjusting the mix screw. if you screw the mix screw all the way in the bike may not stall, the idle passage splits as it enters the carb ,and the mix screw only blocks one  passage , the other remains open and may supply enough fuel to keep the motor running.( the mix screw doesn't turn off the fuel ,it more a fine adjustment)
      When you last did the needle spacer ,you said that you moved the clip on the needle ,this may have been a mistake , using  the washers should  have been enough ,moving the clip may have sent to much fuel and could be the reason your sparkplug is still covered in soot(unburned fuel) Your trying to compensate by using a 47.5 pilot jet (you should be able to run the stock 52.5) the real problem I believe is the needle jet is to high. when your on the road your at a partial throttle setting ,above idle , and below 3/4 to full throttle , your speed is  controlled by the needle jet /jet needle,(the main source of your fuel)  and that's your problem. If you move the needle clip back to it's stock position and the plug stops sooting ,  but you develop a mid range surging it won't be because of the needle , it will be because your pilot jet is now to small.The reason you close the mix screw in and hear no difference in rpm is because the pilot jet is so small the throttle plate is open farther than normal to allow normal idle speed ,and this lifts the needle giving the carb the extra fuel it needs to idle ,closing the  mix screw only cuts off a part of the small amount of fuel coming from the very small pilot jet and doesn't do much to effect the rpm.(you can't hear the difference)

Castheandra

When you turn the air screw in (clockwise), does it "seat" or tighten? If it doesn't seat your screw could be stripped and this could be a source of an airleak.