8MM retainers/locks on 5/16 valves: OK or not OK?

I can't say I have ever paid attention to that dimension. I'll have to check a few locks and valves at some time after I get back home. That matches what you found in post #19.

As long as there is spring tension on the retainer, the locks would be held solidly up against the upper edge of the valve's groove. The only time they could possibly move down in the groove is if some severe spring vibration actually made the spring momentarily contract a tiny bit. What valve spring arrangement are you using?