That certainly explains why the valve margins are clean in your post 23.
The valves have been kissing themselves.
Are all the valves like this, are they all the same, some worse than others, or some not worn at all?
I would find it hard to believe they are all like this. How on earth the engine ran at all without snapping valves off.
What valve stem seal was fitted?
Looks like it had a bad combination of valve guide material, and NO lubrication past a positive valve stem seal, which viton is very good at.
Ultimately the valve stem seals would have let
oil through, the engine would have been a heavy smoker, since the seal would have pulled out of round while the engine was running.
It would come down to the cylinder head manufacturer's choice of guide material as the cause.
Gary