Made as simple as possible, but not simpler.
Don’t. Identifiers should be opaque. If you’re given an owl:sameAs assertion from a party you trust, use that.
Is a metadata record “almost” expressed in the same language you used for your filter criteria?
Why would one consider indexing validators? Reuse. The value of reuse seems obvious for structural and semantic specification, i.
Indexing identifiers is key to disambiguating entities. Wikipedia has disambiguation pages.