HoLun, do the umbrella seals show signs of damage? I would say that is either valve float or the retainers are hitting the seals.The galling on the stem looks like the top of the valve guide was closing up, perhaps from contact by the retainer.
I'm told Qualcast stuff is US made but have no experience with their valves. There are 3 grades of keepers. Stock replacement stamped, hardened stamped and hardened/machined.
The stock 2 piece retainers are heavy and can cause valve float at lower rpm than lighter parts.