I do believe that jsonnet (despite all it's real problems) is an underestimated jewel mostly because of the embryonic level of its tooling. An ide/debugger like this could help.
EDIT: some tangentially related work: https://github.com/kubecfg/ursonnet
In fact, I believe it's possible to write code in TypeScript that would be very similar to Jsonnet. TypeScript provides the capability to create compiler plugins, which can restrict the allowed language subset, effectively making it more limited and suitable for configuration
Evaluation is driven "from the output"; by which I mean that only the code that has an effect on the output is evaluated. When an object is rendered (manifested) all it's non-hidfen fields are evaluated and every expression they contain produces a value which is then manifested and recursively only the fields that are manifested have their expressions evaluated.
This ia particularly important when combined the key operation that jsonnet provides: object overlay/merges. When you overlay an object on another object it may shadow a field of the "super" object causing the expression of the super object to not be evaluated.