Quick suggestion:
I feel the terminology "assert validity" could be confusing from an end-user standpoint. The option description and documentation is clear, but the option's name itself could be taken to mean that the user is asserting the validity of the document, not the script. I know it was modeled after existing XProc steps like this one, but something along the lines of "Abort on Validation" or "Strict Validation" might be more clear and user-friendly.
Maybe all scripts should have a "Input validation" option with 3 possible values: "No validation", "Report validation errors" and "Abort on validation errors".
See comment by Sean: