He put them on without looking at the valves?? Pretty hard to miss a burnt valve. Did he wash them prior to installing? If he did a good cleaning job, he would have seen a burnt valve. If he didn't clean them, what else got in your engine.
I don't think you have a very good mechanic.
They should have gotten a thorough cleaning before assy even if they were sealed from factory. You don't know what's in them!!!
I'll say it again...I'm so happy to live an hour away from the Wizard. No way Chuck would install a used head like that without going over every millimeter. Good to see Paul backing up his sale although I'm not surprised.