sample.plist shows same exact errors so I guess it's the config checker and not my config file.
Those two boot picker features were added to OC recently. The 'missing schema' error means validator expected that feature definition but didn't find it in the supplied config.plist. If it's also missing from Sample.plist that could be a quality oversight in the release where the Sample.plist didn't get properly updated. Or you are checking a version of Sample that predates that version of ocvalidate.
You are right that it could also be an issue with the config checker, but I read about that specific feature (schema) introduced in recent release notes, so it's Sample.plist that's out of date.
When parameters are missing from config.plist, I expect that OC will default them to the "safe" value defined in doc/Configuration.pdf. (Unless there's also a QC issue there.)
Look in doc/Configuration.pdf for the details and set your config accordingly.
Regarding OpenCore Configurator: Keep in back of your mind when using any 3rd party configurator that it needs to be kept in sync with whatever release of OC under is under consideration. If these get out of sync, mayhem can result. Also notice that the developer of configurator may introduce local ideas (or errors) about needed config that conflict with OC. So any helper should be thought of as helping up to the point that it introduces its own bugs.
We are all standing on trust.
In a free community, when you come across bugs/issues, there's also responsibility to help get them resolved. So you might give the developers a heads-up over the project Github. Where things get really sticky is when bug reports are themselves bugs... There's no way to win.