What do you mean by "explain data"? But yes if you returns a non-string value, it works as you expect (I think).

https://clojuredocs.org/clojure.spec.alpha/explain-data

Currently no, but in one project needed richer explanations and did an ugly hack to enable that. I think we should support that at library level. E.g. one could return {:message "so wrong", :level "warning"}

Could you write an issue of this?