Indeed, the pitting is an indication of mal-adjustment. Once its this far though it is too late. Most importantly with exhaust valves is the need for them to close completely and correctly. The valves, especially the exhaust, rely on contact with the valve seat to cool by convection, in this case, that valve, whether it was a weak spring, guide galling, or gross mal-adjustment, the valve stuck open, causing hot gases to constantly blow by, thus overheating the valve, thus burning it, first causing the pitting then worse case scenario the valve head breaks.

The black on the dome of the cylinder in question in my guess is actually coolant coming from the leaking head gaskets. Not enough to show in the oil but enough to cause carbonization. I don't think there is enough oil to really cause a "coking" as the cylinder walls look to be in too good enough shape.