Your pictures seem good enough to see what is going on. I don't see the square headed drain plug in your muffler that we can see in my
M59 & not sure what that means....
BTW, I didn't remove that plug on mine to see if it would put out some carbon. I just turned it a quarter turn just to prove to myself that it was free, will wait until some day when the muffler is hot to remove it. I'm not expecting much.
Right now my inclination would be to borescope it and follow that with a snake if it seemed necessary.
Our
M59 smokes heavily when starting or changing revs...but burns clean at constant RPM. The EGR valve system doesn't appear to do anything at all.
rScotty